The SVN unit tests change the test backend in setUp(), and restore the

old state in tearDown(). Or, they're supposed to. If a backend isn't

installed (such as subvertpy on Python 3.8+), we raise a SkipTest.

This actually bypasses tearDown(), meaning we keep the old test

settings.
This eventually impacts other tests that depend on a working SVN

backend.
We now handle the management of the backend in setUpClass() and

tearDownClass(), which won't be affected by the SkipTest. This

reduces the work that needs to be done per-test, and ensures we'll end

up with the original state once the tests have completed.

All unit tests pass without subvertpy installed.