This allows makemessages/compilemessages tests in `test_extraction.py`
and `test_compilation.py` to actually run isolated from each other
(unaffected by stray FS objects left by cleanup actions failures, debug
sessions, etc.) and to take advantage of the parallel tests execution
feature like most of the Django test suite.

`test_percents.py` gets slightly refactored to not inherit from the new
machinery which sets up every test case to copy and run under a
temporary tree.

Thanks Alex Morozov for the initial patch.

Thanks ArcTanSusan for the initial patch.

A change of the USE_TZ or TIME_ZONE settings doesn't necessarily require a
change to the active connections' timezones.