Add unit tests for PRESUBMIT.py

Add a unit test program, and mocks, for PRESUBMIT.py. Additionally
add unit tests for two newly added presubmit checks which landed
in http://review.skia.org/668416.

These tests also identified a bug in _CheckReleaseNotesForPublicAPI.
That fix is also in this change.

Bug: skia:14276
Change-Id: I90d86e4d5216067484340e85e31f5a33a3eeba3a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/680017
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Chris Mumford <cmumford@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
3 files changed