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.
This is a backport of just the hgsubversion parts of /r/10952/

Ran unit tests.