Add tests for SkFontMgr::matchFamilyStyle

Ensure that matchFamilyStyle returns nullptr when it should and doesn't
when it shouldn't.

Update SkFontMgr_Mac to pass the tests.

Change-Id: Ia8faa68f9961095454b8c0e8d792073516b33314
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/657658
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Herb Derby <herb@google.com>