tests: Chain up to dbusmock parent class methods
Otherwise dbusmock never gets a chance to clean up its temporary mock system bus. With dbusmock >= 0.30.0, it continues to run, holding stdout/stderr open, until the test times out.
This requires correcting the signature of OopTests.setUp(): it isn't actually a class method.
Resolves: #142 (closed)
Bug-Debian: https://bugs.debian.org/1058116