amqp_client_SUITE: Trim "list_connections" output before parsing it

[Why]
Sometimes, at least in CI, it looks like the output of the CLI is
prepended with a newline, sometimes not. This breaks the check of that
output.

[How]
We just trim the output before parsing it. The parsing already takes
care of trimming internal whitespaces.

deps/rabbit/test/amqp_client_SUITE.erl

@@ -4087,7 +4087,7 @@ list_connections(Config) ->
 
     %% CLI should list AMQP 1.0 container-id
     {ok, StdOut1} = rabbit_ct_broker_helpers:rabbitmqctl(Config, 0, ["list_connections", "--silent", "container_id"]),
-    ContainerIds0 = re:split(StdOut1, <<"\n">>, [trim]),
+    ContainerIds0 = re:split(string:trim(StdOut1), <<"\n">>, [trim]),
     ContainerIds = lists:sort(ContainerIds0),
     ?assertEqual([<<>>, ContainerId0, ContainerId2],
                  ContainerIds),