Fixed ResourceWarning from unclosed SQLite connection.

- backends.tests.ThreadTests.test_default_connection_thread_local:
    on SQLite, close() doesn't explicitly close in-memory connections.

Check out python/cpython#108015.

tests/backends/tests.py

@@ -792,7 +792,8 @@ def runner():
             # closed on teardown).
             for conn in connections_dict.values():
                 if conn is not connection and conn.allow_thread_sharing:
-                    conn.close()
+                    conn.validate_thread_sharing()
+                    conn._close()
                     conn.dec_thread_sharing()
 
     def test_connections_thread_local(self):