In order to run the same tests across both PySVN and Subvertpy, we keep

all our tests in a common class and then subclass that for each backend.

However, the test runner still sees this as a valid test class, given

the test functions in it.
This change fixes that by indicating that the tests class should be

private by prefixing the name with an underscore.  That tells the test

runner to ignore it, without having to set __test__ = False in the

class and __test__ = True in the subclasses.

Ran the unit tests. Verified tests for both backends had run, but that

no tests from the common class were run (previously they'd run and then

show up as skipped).