This updates the Perforce unit tests to build a PerforceClient in

each test where it's needed, rather than in setUp(). This is in

prepration for new tests that will need to handle client construction

differently.
This also cleans up a lot of the assertions to be less verbose and to be

more comprehensive.
We had a lot of combinations of checking instance type, key presence,

and then values, but this is far less useful (less accurate) than just

checking dictionaries for equality. We missed things because we weren't

doing these checks.
It also fixes up diff checking to compare for content and not just a

hash. This will be important for work on supporting alternative diff

tools.

All Perforce unit tests pass.