Summary:
This migration allows `./run-tests.py` to run `hgsubversion` tests. Since hgsubversion
tests are actually python unittests, we don't care about their output at all,
they fail differently. But the behavior of the test suite require us to match
whatever the tests prints. We do this later in the stack.
Depends on D6719886
Test Plan: - check that tests are runnable at least
Reviewers: #sourcecontrol
Differential Revision: https://phabricator.intern.facebook.com/D6719890