More details please?

The pre-upload hook already does this. 

Yes, but I would find it very convenient to do this straight from the test (with an extra commandline).

Alternatively, a really, really simple way to run the preupload hooks without trying to upload would help.

I'm starting to add --update to the chromite unittest command line.

Whenm I'm done, two commands will update config_dump.json, and waterfall_layout_dump.txt.

  run_tests --update
  cbuildbot/chromeos_config --update

We'll also have an easy hook for other tests to use if they want similar behavior.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/9b91a54faf07862759ad96829dad94a1971e05ce

commit 9b91a54faf07862759ad96829dad94a1971e05ce
Author: Don Garrett <dgarrett@google.com>
Date: Thu Feb 02 04:27:53 2017

cros_test_lib: Add -u and --update to unittests.

Add --update to all chromite unittests to indicate that generated data
files should be updated. Currently, this can be used to update both
config_dump.json and waterfall_layout_dump.txt.

Both of these commands will update both of those files if needed:
  cbuildbot/run_tests --update
  cbuildbot/chromeos_config_unittest --update

If --update is not present, both tests will still fail if the files in
question are out of date.

BUG= chromium:684918 
TEST=run_tests (with/without -u and with/without modified dump files).

Change-Id: I7e1f1a0c966023ccf507754dea4577638035410a
Reviewed-on: https://chromium-review.googlesource.com/435570
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>

[modify] https://crrev.com/9b91a54faf07862759ad96829dad94a1971e05ce/cbuildbot/chromeos_config_unittest.py
[modify] https://crrev.com/9b91a54faf07862759ad96829dad94a1971e05ce/lib/cros_test_lib.py