Fix running Subversion unit tests.
Fix running Subversion unit tests.

Recent changes to the test runner setup ended up causing Subversion unit
tests to fail to run. The `__test__ = False` on the common test class
was respected, but carried through to the subclasses, preventing any
PySVN or Subvertpy tests from running.

This sets `__test__ = True` on both subclasses, allowing them to run
again.

Testing Done:
Ran the Subversion unit tests. Verified they could now be run.

Reviewed at https://reviews.reviewboard.org/r/12146/

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