tests: drop hard-coded list of tests The list of out of date, missing 'test_helpers' and 'comprehensive/test_custom_layout'. Instead, use the discover functionality introduced in Python 2.7, and available for Python 2.6 and earlier from the 'unittest2' backport. Tested by invoking 'run.py' both with and without '-A' in Python 2.6 & 2.7, and ensuring that passing comprehensive tests as arguments continues to work. As a minor (but welcome) side-effect, this should restore the ability to test hgsubversion under demandimport; previously, test_util was imported before we enabled demandimport, so it didn't affect most of Mercurial. Since unittest2 (and unittest) do define SkipTest, we can remove the earlier import, restoring the likely originally intended testing mode.