Fix hgsubversion test case skipping.
Review Request #10977 — Created March 27, 2020 and submitted
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
setUpClassand 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.