tests: add test for hg-test-mode emacs code
This is just coverage for the compilation-mode support, but that was
enough of a hassle that I wanted to have it covered somehow. Test
methodology is _extremely_ cargo-culted from the test for
compilation-mode in emacs, so I still have no idea what I'm doing.
Differential Revision: https://phab.mercurial-scm.org/D6003