Update: My testing found that the /usr/local/autotest/results directory is cleared each time the test is run. So, we should not store temporary files there either.  Initial testing shows that /usr/local/autotest/ directory contents are safe from reboots and subsequent test runs. So this may be a good candidate. Need to determine best way to get path to /usr/local/autotest/.  Will try using os.path.pardir(self.tmpdir) to get it, since ../bin/test.py seems to indicate that self.tmpdir points to /usr/local/autotest/tmp.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/3e65d6c0c4af7085037a43d2accff14e57be4176

commit 3e65d6c0c4af7085037a43d2accff14e57be4176
Author: Scott Cunningham <scunningham@chromium.org>
Date: Fri Apr 15 14:44:31 2016

Store temporary perf data in local autotest directory.

Problem: Longevity_Tracker.py stores the temporary perf data in the system
/tmp/ directory during the test run. The problem with this is that, if the
device crashes during the test, the data in the /tmp/ directory is often lost.

Solution: Create the temporary perf file in the /usr/local/autotest/
directory. When the test exits, copy the temporary file to 'perf.csv' stored
in the ../results directory, where AutoTest will upload it with the rest of
the test results.

BUG= chromium:603226 
TEST=test_that <IP> f:.*longevity_Tracker/control.longevity_2

Change-Id: I20a2e6717c67002e55822aad9ffed14abba557d3
Reviewed-on: https://chromium-review.googlesource.com/339331
Commit-Ready: Scott Cunningham <scunningham@chromium.org>
Tested-by: Scott Cunningham <scunningham@chromium.org>
Reviewed-by: Scott Cunningham <scunningham@chromium.org>

[modify] https://crrev.com/3e65d6c0c4af7085037a43d2accff14e57be4176/client/site_tests/longevity_Tracker/longevity_Tracker.py