Do not make RunPostTestChecks inherit from TestCase

Aside from the coinciding setUp and tearDown names, it has nothing to do
with a TestCase.

Signed-off-by: mulhern <[email protected]>

testlib/infra.py

@@ -597,7 +597,7 @@ def __str__(self):
         return self.value
 
 
-class RunPostTestChecks(unittest.TestCase):
+class RunPostTestChecks:
     """
     Manage running post test checks
     """