The git-p4 support in GitClient lacked any real testing, which meant

it regressed with Python 3. This also made it harder to iterate on. We

had some fantastic work in recent contributions to extend the features

of git-p4 diffs, but we couldn't reasonably ask for any unit tests to be

written to prove the support works.
This change introduces unit tests that cover get_repository_info() and

diff() for git-p4. It avoids duplicating a lot of the core diff tests,

but does cover all the Perforce-specific operations and multi-commit

review requests.
There's also a bit of cleanup to related tests in this file, to keep

naming consistent.
This is a precursor to work for optimizing and future-proofing this

code.

Unit tests pass.