tests: add missing test cleanup

skip_kernel_validation_patch was not stopped, and keeps reference to
some objects.

Author: marmarek

qubes/tests/__init__.py

@@ -527,6 +527,10 @@ def kernel_validator_patched(obj, key, value):
         )
         self.skip_kernel_validation_patch.start()
 
+    def tearDown(self):
+        self.skip_kernel_validation_patch.stop()
+        super().tearDown()
+
     def cleanup_gc(self):
         # remove cached references to Qubes() object
         qubes.api.internal.SystemInfoCache.cache_for_app = None