Update the SOS unit tests to use build_client() and clean up tests.
Update the SOS unit tests to use build_client() and clean up tests.

This updates the SOS unit tests to build a `SOSClient` 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.

Testing Done:
SOS unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/12510/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools