This updates the Mercurial unit tests to build a MercurialClient 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.
Given the work being done already in each function, I felt this was the

best time to clean this code up. It's caught some missing state we

weren't checking.

Mercurial unit tests pass.