Also adds a new canned_presubmit_checks_test.py and supporting mocks,
based on the PRESUBMIT_*.py under chromium/src.

If this is OK, there are subsequent CLs for removing the original code
from chromuim/src and infra/infra, and having their PRESUBMIT scripts
just reference this canned check instead.

Change-Id: I67dfb7ac0b4cdc36bd62ec2dc062ca5c78c2244e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2805268
Commit-Queue: Sean McCullough <seanmccullough@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>