Fixed leaking file descriptors in unit tests

These weren't any issue in Linux, but Windows doesn't like it when you
try to delete a still-opened file.

tests/test_blendfile_loading.py

@@ -293,6 +293,7 @@ class BlendFileCacheTest(AbstractBlendFileTest):
         self.tpath = pathlib.Path(self.tdir.name)
 
     def tearDown(self):
+        super().tearDown()
         self.tdir.cleanup()
 
     def test_open_cached(self):