Make concurrent runs of FileBackendTest independent

Make each run of the FileBackendTest be for a different "wiki"
so you could have multiple runs at the same time without them
interfering with each other.

There's been intermittent failures on jenkins for this test. I'm
not sure if its some race condition (which wouldn't really make
sense given each run should be independent I thought), but this
can't hurt.

Change-Id: I3b639f1c783efc140028c3acdce35ef3f58bf66f
Merge "Make concurrent runs of FileBackendTest independent"