qa/cephfs: don't inherit CephFSTestCase in CapTester

qa/cephfs: don't inherit CephFSTestCase in CapTester

Inheritting CephFSTestCase in CapTester just for methods assertEqual()
and assertIn() from class unittest.TestCase is odd and heavy-weight.
Don't inherit CephFSTestCase and use simple assert instead.

Reference: https://github.com/ceph/ceph/pull/50882#discussion_r1160611549.

To avoid code duplication, a couple of similar methods have been added
instead.

Signed-off-by: Rishabh Dave <ridave@redhat.com>