A test failure was reported in Daily CI (test-centos7-tls).
`CKQUORUM detects failover authorization cannot be reached`.
```
CKQUORUM detects failover authorization cannot be reached: FAILED:
Expected 'invalid command name "OK 4 usable Sentinels. Quorum and failover authorization can be reached"' to match '*NOQUORUM*'
```
It seems that current sentinel does not confirm that the other
sentinels are actually `down`, and then check the quorum.
It at least take 3 seconds on my machine, and we can see there
will be a timing issue with the hard code `after 5000`.
In this commit, we check the response of `SENTINEL SENTINELS mymaster`
to ensure that other sentinels are actually `down` in the view the
current sentinel. Solve the timing issue due to sentinel monitor mechanism.
7e5ded2ad0