Fix hgsubversion test case skipping.
Fix hgsubversion test case skipping.

The way that hgsubversion tests were skipping was inefficient and
broken, causing a bunch of failed tests in other suites due to an
incorrect cwd restoration. This change improves the way that
hgsubversion tests are skipped, doing all the existence checks inside
`setUpClass` and avoiding any expensive operations if the test suite is
going to be skipped.

Testing Done:
Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/10977/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools