mxs2456_trx_replay_cap assumes a promotion order for servers

The test_replay_failure section in mxs2456_trx_replay_cap assumes that the servers are promoted in the order they are declared. However, it does not check that failover has actually happened before proceeding to block the next node. This causes the test to fail as it assumes the third failover would be rejected but it ends up working as there is only two replay attempts. The right thing to do would be to check that failover has happened before proceeding to block the next node.