rabbit_stream_queue_SUITE: Wait for replicas in shrink_coordinator_cluster/1

[Why]
In CI, we sometimes get a failure when we try to forget node 3. The
CLI doesn't report the nature of the error unfortunately.

I suppose it's related to the fact that node 3 is stopped and forgotten
before all three replicas were ready when the stream queue was declared.
This is just a guess though and have no proof that it is the actual
error.

[How]
We wait for the replicas after declaring the stream queue.

(cherry picked from commit 0fb74ba)

Author: dumbbell

deps/rabbit/test/rabbit_stream_queue_SUITE.erl

@@ -722,10 +722,12 @@ shrink_coordinator_cluster(Config) ->
         rabbit_ct_broker_helpers:get_node_configs(Config, nodename),
     Q = ?config(queue_name, Config),
 
-
     ?assertEqual({'queue.declare_ok', Q, 0, 0},
                  declare(Config, Server0, Q, [{<<"x-queue-type">>, longstr, <<"stream">>}])),
 
+    %% Wait for the replicas to be ready before stopping a node.
+    check_leader_and_replicas(Config, [Server0, Server1, Server2]),
+
     ok = rabbit_control_helper:command(stop_app, Server2),
     ok = rabbit_control_helper:command(forget_cluster_node, Server0, [atom_to_list(Server2)], []),