Refactor extension tests to use less Mock()

Review Request #7909 — Created Jan. 25, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.8.x

Reviewers

The extension tests now use a ExtensionManager subclass that provides
the entry points (which are faked via new utility classes,
FakeEntryPoint, which mocks the entry point, and FakeDistribution,
which mocks the distribution metadata) at initialization time. This
allows futher tests of extension managers to be more succinct and
robust.

Ran unit tests.