It seems that the failure might be caused here:
chromite-0.0.2-r3862: AssertionError: Expected to find call(('gclient', 'config', '--spec', "solutions = [{'custom_deps': {},\n  'custom_vars': {},\n  'deps_file': '.DEPS.git',\n  'managed': True,\n  'name': 'src',\n  'url': 'https://chromium.googlesource.com/chromium/src.git@refs/tags/41.0.2270.0'}]\ntarget_os = ['chromeos']\ncache_dir = '/tmp/b/git-cache'\n"), cwd='/work/chrome') in any of:
chromite-0.0.2-r3862: call(['gclient', 'config', '--spec', "solutions = [{'custom_deps': {},\n  'custom_vars': {},\n  'deps_file': '.DEPS.git',\n  'managed': True,\n  'name': 'src',\n  'url': 'https://chromium.googlesource.com/chromium/src.git@refs/tags/41.0.2270.0'}]\ncache_dir = '/tmp/b/git-cache'\ntarget_os = ['chromeos']\n"], cwd='/work/chrome')

Note how on the 2nd version target_os comes after the cache, while in the expectation it comes before cache entry. So maybe this is causing our failures

chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] GclientWrappersTest.testRevert ... ok
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] GclientWrappersTest.testSync
chromite-0.0.2-r3862: Test gclient.Sync() without optional arguments. ... ok
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] GclientWrappersTest.testSyncWithOptions
chromite-0.0.2-r3862: Test gclient.Sync() with optional arguments. ... ok
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] GclientWrappersTest.testSyncWithRunArgs
chromite-0.0.2-r3862: Test gclient.Sync() with run_args. ... ok
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpec
chromite-0.0.2-r3862: Test WriteConfigFile with chrome checkout and no revision. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecDepsResolution
chromite-0.0.2-r3862: Test BuildspecUsesDepsGit at release thresholds. ... ok
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecWithGclientTemplate
chromite-0.0.2-r3862: Test WriteConfigFile with chrome checkout with a gclient template. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecWithGitHash
chromite-0.0.2-r3862: Test WriteConfigFile with chrome checkout at a given git revision. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecWithGitHashNoManaged
chromite-0.0.2-r3862: Like testChromeSpecWithGitHash() but with "managed" sets to False. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecWithGitHead
chromite-0.0.2-r3862: Test WriteConfigFile with chrome checkout at a given git revision. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecWithReleaseTag
chromite-0.0.2-r3862: Test WriteConfigFile with chrome checkout at a given release tag. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromeSpecWithReleaseTagDepsGit
chromite-0.0.2-r3862: Test WriteConfigFile with chrome checkout at a given release tag. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromiumSpec
chromite-0.0.2-r3862: Test WriteConfigFile with chromium checkout and no revision. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromiumSpecNotUseCache
chromite-0.0.2-r3862: Test WriteConfigFile with chromium checkout and no revision. ... ok
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromiumSpecWithGitHash
chromite-0.0.2-r3862: Test WriteConfigFile with chromium checkout at a given git revision. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromiumSpecWithGitHead
chromite-0.0.2-r3862: Test WriteConfigFile with chromium checkout at a given git revision. ... FAIL
chromite-0.0.2-r3862: [chromite.lib.gclient_unittest] TestGclientWriteConfigFile.testChromiumSpecWithReleaseTag
chromite-0.0.2-r3862: Test WriteConfigFile with chromium checkout at a given release tag. ... FAIL

Also interesting that the test not using the cache is among the ones passing.

Could we modify gclient.py to append the target_os before the cache_dir (rather than after as done by https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1037920/7/lib/gclient.py) so the expectations match?    - says the ARC++ gardener who is out of his depth here...

#6 is worth trying...

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/5295eaac6d224a44bac89b9b919bff2532ffcda4

commit 5295eaac6d224a44bac89b9b919bff2532ffcda4
Author: Josh Horwich <jhorwich@chromium.org>
Date: Wed May 02 20:30:46 2018

Reorder target_os / cache_dir in gclient spec

Unit tests on builders were failing due to mis-ordered spec vs.
expectatoin. This moves target_os before cache_dir to align with
expectations.

BUG=chromium:838847
TEST=chromite/lib/gclient_unittest

Change-Id: I7a4cc57a3446873cd993ef29ea895d657503f4b3
Reviewed-on: https://chromium-review.googlesource.com/1040305
Trybot-Ready: Josh Horwich <jhorwich@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Agi Csaki <agicsaki@chromium.org>
Commit-Queue: Josh Horwich <jhorwich@chromium.org>
Tested-by: Josh Horwich <jhorwich@chromium.org>

[modify] https://crrev.com/5295eaac6d224a44bac89b9b919bff2532ffcda4/lib/gclient.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/039b813669fae464174b6ba21533779ef9c0c640

commit 039b813669fae464174b6ba21533779ef9c0c640
Author: chromite-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com <chromite-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Wed May 02 22:24:19 2018

Roll src/third_party/chromite/ 9dea5038c..5295eaac6 (1 commit)

https://chromium.googlesource.com/chromiumos/chromite.git/+log/9dea5038cdf1..5295eaac6d22

$ git log 9dea5038c..5295eaac6 --date=short --no-merges --format='%ad %ae %s'
2018-05-02 jhorwich Reorder target_os / cache_dir in gclient spec

Created with:
  roll-dep src/third_party/chromite
BUG=chromium:838847


The AutoRoll server is located here: https://chromite-chromium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


TBR=chrome-os-gardeners@chromium.org

Change-Id: I48c42f1dab561e0fc5f8c18287dbd0b7acb6954f
Reviewed-on: https://chromium-review.googlesource.com/1040770
Reviewed-by: Chromite Chromium Autoroll <chromite-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Commit-Queue: Chromite Chromium Autoroll <chromite-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#555578}
[modify] https://crrev.com/039b813669fae464174b6ba21533779ef9c0c640/DEPS

I believe the above (comment #8 and comment #9) changes should clear this error.

going by the comments on the CL, we didn't actually fix the bad tests, just worked around them.  can we fix the actual tests ?

Agreed, now that we have a workaround, can we make it robust?

Just as a note, that fix seems to have worked.