test(cm): add sleep time for multicore scheduling (#6164)

apps/emqx/test/emqx_cm_SUITE.erl

@@ -180,6 +180,7 @@ t_open_session_race_condition(_) ->
     exit(Winner, kill),
     receive {'DOWN', _, process, Winner, _} -> ok end,
     ignored = gen_server:call(emqx_cm, ignore, infinity), %% sync
+    timer:sleep(100),
     ?assertEqual([], emqx_cm:lookup_channels(ClientId)).
 
 t_discard_session(_) ->