Fix presubmit.py tests.

Presubmit tests in chromium-build have been broken since https://codereview.chromium.org/23033018.

This fixes that, and adds some info about how to change the test data.

Review URL: https://codereview.chromium.org/142243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/chromium-build@245717 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed