Silence ResourceWarning when testing that the file destructor closes the file.

Lib/test/test_io.py

@@ -461,6 +461,7 @@ class IOTest(unittest.TestCase):
             def flush(self):
                 record.append(3)
                 super().flush()
+        with support.check_warnings(('', ResourceWarning)):
             f = MyFileIO(support.TESTFN, "wb")
             f.write(b"xxx")
             del f