Fix redis-cli cluster add-node race in cli.tcl (#11349)

There is a race condition in the test:
```
*** [err]: redis-cli --cluster add-node with cluster-port in tests/unit/cluster/cli.tcl
Expected '5' to be equal to '4' {assert_equal 5 [CI 0 cluster_known_nodes]} proc ::test)
```

When using cli to add node, there can potentially be a race condition
in which all nodes presenting cluster state o.k even though the added
node did not yet meet all cluster nodes.

This comment and the fix were taken from #11221. Also apply it in several
other similar places.