test(shared_sub): attempt to reduce test flakiness

https://github.com/emqx/emqx/actions/runs/5122717037/jobs/9215439237#step:8:565
```
%%% emqx_shared_sub_SUITE ==> t_local_fallback: FAILED
%%% emqx_shared_sub_SUITE ==> {{badmatch,<<"not yet?">>},
 [{emqx_shared_sub_SUITE,t_local_fallback,1,
                         [{file,"/__w/emqx/emqx/source/apps/emqx/test/emqx_shared_sub_SUITE.erl"},
                          {line,669}]},
```

apps/emqx/test/emqx_shared_sub_SUITE.erl

@@ -666,7 +666,7 @@ t_local_fallback(Config) when is_list(Config) ->
     {true, UsedSubPid1} = last_message(<<"hello1">>, [ConnPid1]),
 
     rpc:call(Node, emqx, publish, [Message2]),
-    {true, UsedSubPid2} = last_message(<<"hello2">>, [ConnPid1]),
+    {true, UsedSubPid2} = last_message(<<"hello2">>, [ConnPid1], 2_000),
 
     emqtt:stop(ConnPid1),
     stop_slave(Node),