firmware_TouchpadMTB: calculate the sample standard deviation

This CL prints the sample standard deviations so that it is easy to
observe the consistency across validators and firmwares. An example
of the summary looks like:

Test Summary (by gesture)  :     fw_11.26           fw_11.23
                                 mean  ssd  count   mean  ssd  count
--------------------------------------------------------------------
one_finger_tracking
  CountTrackingIDValidator :     1.00  0.00     4   0.75  0.50     4
...
finger_crossing
  NoGapValidator           :     0.77  0.42     4   0.92  0.17     4
...

BUG=chromium-os:35752
TEST=Run the command on the host.
$ cd .../autotest/files/client/site_tests/firmware_TouchpadMTB
$ python firmware_summary.py tests/logs/
Observe that it displays the scores, the sample standard deviations,
and counts for every firmware.

Also confirm that all unit tests should pass.
$ python tests/firmware_summary_unittest.py

Change-Id: Id4c382562f7c78d5f4e1919478404d1e667b1f4b
Reviewed-on: https://gerrit.chromium.org/gerrit/41014
Reviewed-by: Charlie Mooney <charliemooney@chromium.org>
Commit-Queue: Joseph Shyh-In Hwang <josephsih@chromium.org>
Tested-by: Joseph Shyh-In Hwang <josephsih@chromium.org>
2 files changed