DiskFileMixin and DiskFileManagerMixin has almost
identical setUp() and tearDown() methods, and both
inherit BaseDiskFileTestMixin, so this moves the common
code into the abstract superclass.
Also remove repeated declaration of vars in
test_diskfile.py:run_quarantine_invalids
and a duplicated qualified import in obj/test_server.py
Change-Id: Id99ba151c7802c3b61e483a7e766bf6f2b2fe3df