tests: use interfaceutil in simplestorerepo
Without it, we crash at module import time.
This has likely been busted since
856f381ad74b.
Differential Revision: https://phab.mercurial-scm.org/D3884
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.