test: make TestBase inherit unittest.TestCase

We want to extend our base-class to support extensions to
unittest.TestCase, so make sure we inherit from it.

Adjust all callers to no longer inherit from TestCase, since this is now
done automatically by TestBase.

test/src/test_pylint.py

@@ -10,7 +10,7 @@ from .. import test
 
 
 @unittest.skipUnless(test.TestBase.have_test_checkout(), "no test-checkout access")
-class TestPylint(test.TestBase, unittest.TestCase):
+class TestPylint(test.TestBase):
     def test_pylint(self):
         #
         # Run `pylint` on all python sources. We simply use `find` to locate