improve ThreadingTestCases.test_FSTestCases_in_separate_dirs.
improve ThreadingTestCases.test_FSTestCases_in_separate_dirs.

It's now called "test_cases_in_separate_dirs" and uses tests defined on
self.__class__ rather than the base FSTestCase suite.

pombreda/pyfilesystem,piranna/pyfilesystem,PyFilesystem/pyfilesystem,pmitros/pyfs,zopyx/pyfilesystem,pmitros/pyfs,flying-circus/pyfilesystem,Konubinix/pyfilesystem,rivenz/pyfilesystem