match: add tests for visitdir functionality There are a few cases that we could have done better with some additional logic; I tried to annotate these when I noticed them, but may have missed some. The tests are not exhaustive; there are certainly some patterns that I didn't test well, and many that I didn't test at all. The primary motivation was to get coverage on visitdir so that I can cover identical cases in a similar method I'm working on, to make sure that this new method behaves the same (or better). Differential Revision: https://phab.mercurial-scm.org/D4128