Inherit from DBusTestCase directly in DNS provider test

This allows the DBus mock server to be properly torn down once the
tests are done running. DBusTestCase inherits from unittest.TestCase,
so it remains functionally identical.

test/main/test_dns_server_provider.py

@@ -1,6 +1,5 @@
 from ipaddress import IPv4Address
 from typing import Callable
-from unittest import TestCase
 from unittest.mock import patch, Mock
 
 from dbusmock import DBusTestCase
@@ -18,10 +17,10 @@
         Gio.DBusConnectionFlags.MESSAGE_BUS_CONNECTION | Gio.DBusConnectionFlags.AUTHENTICATION_CLIENT,
     )
 )
-class TestDNSServerProvider(TestCase):
+class TestDNSServerProvider(DBusTestCase):
     @classmethod
     def setUpClass(cls) -> None:
-        DBusTestCase.start_system_bus()
+        cls.start_system_bus()
 
     def test_resolver(self):
         with open("/tmp/resolv.conf", "w") as f: