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

contrib/python3-whitelist

@@ -106,6 +106,7 b' test-context.py'
 test-contrib-check-code.t
 test-contrib-check-commit.t
 test-contrib-dumprevlog.t
+test-contrib-emacs.t
 test-contrib-perf.t
 test-contrib-relnotes.t
 test-contrib-testparseutil.t