tests/test-contrib-emacs.t

py3: add test-contrib-emacs.t to passing tests list

I installed emacs on the server running buildbot and the test started passing on
Python 3. Lets add it to the list of passing test.

Differential Revision: https://phab.mercurial-scm.org/D6500

#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)