tests: Clean up the temporary dirs post-run.

TemporaryDir only deletes empty dirs (not done by its dtor because it
tries to keep the temporary files available on error exit).

Also change FreeCacheTest::MockFreeSpaceChecker to be static.

Test: Run recovery_unit_test on marlin. Check /data/local/tmp post-run.
Change-Id: I1bd54eb840e3094b4f22ee84c059eec2998773bf