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/abstract_test.py

@@ -17,5 +17,7 @@ class AbstractBlendFileTest(unittest.TestCase):
         self.bf = None
 
     def tearDown(self):
+        if self.bf is not None:
+            self.bf.close()
         self.bf = None
         blendfile.close_all_cached()

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):