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}]}]
```
796c04e7a8