channel_source_SUITE: Wait for server channel

... in `undefined_channel_source` testcase. This testcase fails
transiently and it looks like a timing issue:

channel_source_SUITE > non_parallel_tests > undefined_channel_source
    #1. {error,
            {{badmatch,[]},
	     [{channel_source_SUITE,undefined_channel_source1,1,
                  [{file,"test/channel_source_SUITE.erl"},{line,138}]},
              {rpc,'-handle_call_call/6-fun-0-',5,
                  [{file,"rpc.erl"},{line,197}]}]}}

(cherry picked from commit 43037ce)

Author: dumbbell

test/channel_source_SUITE.erl

@@ -135,8 +135,19 @@ undefined_channel_source(Config) ->
 undefined_channel_source1(_Config) ->
     ExistingChannels = rabbit_channel:list(),
     {_Writer, _Limiter, ServerCh} = rabbit_ct_broker_helpers:test_channel(),
-    [ServerCh] = rabbit_channel:list() -- ExistingChannels,
+    wait_for_server_channel(ExistingChannels, ServerCh, 60),
     [{source, undefined}] = rabbit_channel:info(ServerCh, [source]),
     _ = rabbit_channel:source(ServerCh, ?MODULE),
     [{source, ?MODULE}] = rabbit_channel:info(ServerCh, [source]),
     passed.
+
+wait_for_server_channel(ExistingChannels, ServerCh, 0) ->
+    [ServerCh] = rabbit_channel:list() -- ExistingChannels;
+wait_for_server_channel(ExistingChannels, ServerCh, Attempts) ->
+    case rabbit_channel:list() -- ExistingChannels of
+        [ServerCh] ->
+            ok;
+        _ ->
+            timer:sleep(1000),
+            wait_for_server_channel(ExistingChannels, ServerCh, Attempts - 1)
+    end.