brillo_Gtests: allow manual filtering of tests

You can run specific unittests by doing:
	$ ./site_utils/test_droid.py emulator-5554 brillo_Gtests \
		--args='tests=*metrics*'

This runs any unittest found matching that pattern.

BUG=b:26798540
TEST=`./test_droid.py emulator-5554 brillo_Gtests` runs all tests
TEST=`./test_droid.py emulator-5554 brillo_Gtests --args='whitelist=True'` runs whitelisted tests
TEST=`./test_droid.py emulator-5554 brillo_Gtests --args='filter=*metrics*'` runs metrics tests
TEST=`./test_droid.py emulator-5554 brillo_Gtests --args='tests=/some/test'` runs only /some/test

Change-Id: Ib4deb8cf290486c9d8ed0a1c5f91e055ee1a812c
Reviewed-on: https://chromium-review.googlesource.com/329030
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
3 files changed