Fixed catching warning, as per python-pillow#8505

Author: radarhere

Tests/test_file_avif.py

@@ -182,6 +182,8 @@ def finish(self) -> None:
     def test_no_resource_warning(self, tmp_path: Path) -> None:
         with Image.open(TEST_AVIF_FILE) as im:
             with warnings.catch_warnings():
+                warnings.simplefilter("error")
+
                 im.save(tmp_path / "temp.avif")
 
     @pytest.mark.parametrize("major_brand", [b"avif", b"avis", b"mif1", b"msf1"])