The extensions test suite is old, and maintaining tests has been

increasingly difficult. More tests are coming, and to make it easier to

continue adding and refining tests, this change splits it into several

different test suites.
It also moves the old djblets.extensions.test module (a container for

some test-related modules, but not unit tests) into an apps directory

within the new djblets.extensions.tests.
The code hasn't really changed, aside from the addition of some missing

docstrings and path updates.

Unit tests pass.