cluster_minority_SUITE: Fix race in remove_node_when_seed_node_is_leader

[Why]
When node A becomes the leader, it still takes a few exchanges with
followers to allow cluster changes again.

Concurrently, the testcase blocks traffic between A and other nodes to
simulate a network partition. If this happens after A becomes the leader
but before cluster changes are permitted again, the testcase will never
succeed and will eventually abort with:

    Case: cluster_minority_SUITE:remove_node_when_seed_node_is_leader
    Reason: {error,
                {{awaitMatch,
                     [{module,cluster_minority_SUITE},
                      {expression, "..."},
                      {pattern,"ok"},
                      {value,
                          {error,69,<<"Error:\ncluster_change_not_permitted">>}}]},

[How]
Before blocking traffic, we wait for cluster changes to be permitted
again on node A.

Author: dumbbell

deps/rabbit/test/cluster_minority_SUITE.erl

@@ -395,6 +395,10 @@ remove_node_when_seed_node_is_leader(Config) ->
     ct:pal("Member A state: ~0p", [Pong]),
     case Pong of
         {pong, leader} ->
+            ?awaitMatch(
+               {ok, #{cluster_change_permitted := true}, _},
+               rabbit_ct_broker_helpers:rpc(Config, A, ra, member_overview, [AMember]),
+               60000),
             ?awaitMatch(
                ok,
                rabbit_control_helper:command(