test(dm): unstable test validator_basic failover

[buchuitoudegou]

https://ci2.pingcap.net/blue/organizations/jenkins/dm_ghpr_integration_test/detail/dm_ghpr_integration_test/8862/pipeline

We tried to trigger two flush checkpoints, but only one occurred.

possible root cause:

[buchuitoudegou]

/area dm /assign

[buchuitoudegou]

/component test