Ape xUnit more closely to remove ordering constraints on test subs:

Ape xUnit more closely to remove ordering constraints on test subs:
* Add setup and teardown methods, called before and after every test sub.
* In setup, make a fresh repo; in teardown, throw it out.
* Extract runtests method and define default test methods at top.
* Move reflection routines near the xUnit-style subs they support.

Adapt existing test subs to run independently:
* In test_manual_add_and_commit(), assume a fresh repo.

While here, plan a bit better:
* Check for all modules used by cvs.pm.
* Check for program existence more generally.
* Check that we can rmdir after mkdir.