Add hpack-test-case submodule to test resources.

Issue square/okhttp#931

I think this is the simplest way to keep the repos in sync and avoid buggy maven configs.  It does require a git submodule init && git submodule update after a clone, so maybe there's a better way.
nit: as a general rule, I never use HashSet. Only LinkedHashSet. In this particular case it's okay, but I'd prefer always using LHS and then never having to think about it.

LHS is deterministic; HashSet is not. If you always use the deterministic one, when your tests are flakey there's one less thing to think about.