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
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support
Ran 1 tests, 1 results as expected (*) (glob)