This updates the Bazaar unit tests to build a BazaarClient 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.
The test suite temporarily implements a wrapping build_client()

function that sets the appropriate bzr/brz binary as the default for

the client. When the new dependency code is ready, this function will be

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

more specific. 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. Given the work

being done already in each function, I felt this was the best time to

clean this code up.

All unit tests pass.