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/test.py

@@ -9,12 +9,13 @@ import os
 import subprocess
 import sys
 import tempfile
+import unittest
 
 import osbuild
 from osbuild.util import linux
 
 
-class TestBase():
+class TestBase(unittest.TestCase):
     """Base Class for Tests
 
     This class serves as base for our test infrastructure and provides access