Fixed ResourceWarning from unclosed SQLite connection on Python 3.13+.

- backends.sqlite.tests.ThreadSharing.test_database_sharing_in_threads
- backends.tests.ThreadTests.test_default_connection_thread_local:
    on SQLite, close() doesn't explicitly close in-memory connections.
- servers.tests.LiveServerInMemoryDatabaseLockTest
- test_runner.tests.SQLiteInMemoryTestDbs.test_transaction_support

Check out https://github.com/python/cpython/pull/108015.

tests/backends/tests.py

@@ -792,7 +792,8 @@ class ThreadTests(TransactionTestCase):
             # 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):