py3: add two passing tests to whitelist found by buildbot
The buildbot found these two new passing tests on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4607
phabricator: mark extension as experimental for now
I don't want us to commit to this having a stable interface just yet.
Differential Revision: https://phab.mercurial-scm.org/D4605