Run SVN unit tests with both PySVN and Subvertpy backends.

The SVN unit tests are no longer simply based on what SVN backend we
have installed on our machine. Instead, we now run two copies of the SVN
tests: One with PySVN, and one with Subvertpy.

If a particular backend is not installed, those tests will be skipped.

Reviewed at https://reviews.reviewboard.org/r/5972/