I don't think there's any reason to use the local backend in tests
instead of using the stricter test backend.
I think we should generally use the test backend in tests and only use
the local backend or git backend when there's a particular reason to
do so (such as in `test_bad_locking` where the on-disk directory
structure matters). But this patch only deals with the simpler cases
where we were only testing with the local backend.