test: fix flaky test

```
 Testing lib.emqx.emqx_cm_SUITE: *** FAILED test case 24 of 31 ***
%%% emqx_cm_SUITE ==> t_stepdown_session_takeover_begin_timeout: FAILED
%%% emqx_cm_SUITE ==>
Failure/Error: ?assertEqual([<0.6635.8>,<0.6520.8>], lists : sort ( emqx_cm : lookup_channels ( ClientId ) ))
  expected: [<0.6635.8>,<0.6520.8>]
       got: [<0.6520.8>,<0.6635.8>]
      line: 309
```

apps/emqx/test/emqx_cm_SUITE.erl

@@ -306,7 +306,7 @@ test_stepdown_session(Action, Reason) ->
     ok = emqx_cm:register_channel(ClientId, Pid1, ConnInfo),
     ok = emqx_cm:register_channel(ClientId, Pid1, ConnInfo),
     ok = emqx_cm:register_channel(ClientId, Pid2, ConnInfo),
-    ?assertEqual([Pid1, Pid2], lists:sort(emqx_cm:lookup_channels(ClientId))),
+    ?assertEqual(lists:sort([Pid1, Pid2]), lists:sort(emqx_cm:lookup_channels(ClientId))),
     case Reason of
         noproc ->
             exit(Pid1, kill),