Fix redis-cli cluster test timing issue (#11887)

This test fails sporadically:
```
*** [err]: Migrate the last slot away from a node using redis-cli in tests/unit/cluster/cli.tcl
cluster size did not reach a consistent size 4
```

I guess the time (5s) of wait_for_cluster_size is not enough,
usually, the waiting time for our other tests for cluster
consistency is 50s, so also changing it to 50s.