test: fix flaky test

We should emit the trace event before replying to callers.

Example failure:

https://github.com/emqx/emqx/actions/runs/8378977952/job/22946318696#step:6:182

```
 =CRITICAL REPORT==== 21-Mar-2024::17:45:37.676024 ===
"check stage" failed: error
{assertMatch,[{module,emqx_ds_storage_bitfield_lts_SUITE},
              {line,270},
              {expression,"? of_kind ( emqx_ds_replication_layer_egress_flush , Trace )"},
              {pattern,"[ # { batch := [ _ , _ , _ ] } ]"},
              {value,[]}]}
Stacktrace: [{emqx_ds_storage_bitfield_lts_SUITE,
                 '-t_atomic_store_batch/1-fun-1-',1,
                 [{file,
                      "/__w/emqx/emqx/apps/emqx_durable_storage/test/emqx_ds_storage_bitfield_lts_SUITE.erl"},
                  {line,270}]},
             {emqx_ds_storage_bitfield_lts_SUITE,t_atomic_store_batch,1,
                 [{file,
                      "/__w/emqx/emqx/apps/emqx_durable_storage/test/emqx_ds_storage_bitfield_lts_SUITE.erl"},
                  {line,249}]}]
```

apps/emqx_durable_storage/src/emqx_ds_replication_layer_egress.erl

@@ -161,12 +161,13 @@ do_flush(
 ) ->
     case emqx_ds_replication_layer:ra_store_batch(DB, Shard, lists:reverse(Messages)) of
         ok ->
-            lists:foreach(fun(From) -> gen_server:reply(From, ok) end, Replies),
-            true = erlang:garbage_collect(),
             ?tp(
                 emqx_ds_replication_layer_egress_flush,
                 #{db => DB, shard => Shard, batch => Messages}
-            );
+            ),
+            lists:foreach(fun(From) -> gen_server:reply(From, ok) end, Replies),
+            true = erlang:garbage_collect(),
+            ok;
         Error ->
             true = erlang:garbage_collect(),
             ?tp(