Add tests for 'ForTesting' presubmit check

The root PRESUBMIT.py contains
_CheckNoProductionCodeUsingTestOnlyFunctions which checks against
including a call to a 'for testing only' method in production code.
That check has not been tested. This CL adds two tests for it.

Additionally, the CL also fixes two issues in the presubmit tests:
 * FilterSourceFile in PRESUBMIT_test_mocks.py did not match the
   production version from tools/depot_tools/presubmit_support.py: the
   production requires the filename to be in the whitelist, while the
   mock version was OK with just not being in the blacklist. The CL
   modifies the mock version to match the production.
 * The test for _CheckAndroidTestJUnitInheritance did not adhere to
   the whitelisted filename pattern (*Test.java). This CL changes the
   names of the fake files to match the pattern.

Bug: 821981
Change-Id: I65edf07ddb2ae26ad7d08ceb7cf4d51b482b5e56
Reviewed-on: https://chromium-review.googlesource.com/966605
Reviewed-by: Daniel Cheng <[email protected]>
Commit-Queue: Vaclav Brozek <[email protected]>
Cr-Commit-Position: refs/heads/master@{#543783}

• PRESUBMIT_test.py[diff]
• PRESUBMIT_test_mocks.py[diff]