From commit b1369a52c24336da2d2d6d5dc6707a7834065d43
This adds the "REQUIRES" directive that allows specifying
preconditions for tests, which allows us to add tests that don't have
to run on all systems.
Now, I don't want to just make all tests specific to an OS or
something, but e.g. a `git` test would be a honkin' great idea, and we
can't ask everyone to have `git`!