qa: fix test_max_allowed_packet (testStorageMySQL)

It started to fail with commit fd007f5d.

neo/tests/storage/testStorageMySQL.py

@@ -96,9 +96,11 @@ class StorageMySQLdbTests(StorageDBTests):
         assert len(x) + EXTRA == self.db._max_allowed_packet
         self.assertRaises(DatabaseFailure, self.db.query, x + ' ')
         self.db.query(x)
+        # Reconnection cleared the cache of the config table,
+        # so fill it again with required values before we patch query().
+        self.db.getNumPartitions()
         # Check MySQLDatabaseManager._max_allowed_packet
         query_list = []
-        query = self.db.query
         self.db.query = lambda query: query_list.append(EXTRA + len(query))
         self.assertEqual(2, max(len(self.db.escape(chr(x)))
                                 for x in xrange(256)))