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: sadly you should explicitly specify "UTF-8" as a 2nd parameter here. Otherwise if you run this on a machine with a different default charset, bad things happen