test(client mgmt api): attempt to fix flaky test

```
%%% emqx_mgmt_api_clients_SUITE ==> msgs_plain_encoding.t_mqueue_messages: FAILED
%%% emqx_mgmt_api_clients_SUITE ==>
Failure/Error: ?assertEqual(79, Count)
  expected: 79
       got: 100
      line: 1303
```

apps/emqx_management/test/emqx_mgmt_api_clients_SUITE.erl

@@ -1193,7 +1193,7 @@ t_mqueue_messages(Config) ->
     ClientId = atom_to_binary(?FUNCTION_NAME),
     Topic = <<"t/test_mqueue_msgs">>,
     Count = emqx_mgmt:default_row_limit(),
-    {ok, _Client} = client_with_mqueue(ClientId, Topic, Count),
+    ok = client_with_mqueue(ClientId, Topic, Count),
     Path = emqx_mgmt_api_test_util:api_path(["clients", ClientId, "mqueue_messages"]),
     ?assert(Count =< emqx:get_config([mqtt, max_mqueue_len])),
     AuthHeader = emqx_mgmt_api_test_util:auth_header_(),
@@ -1244,14 +1244,16 @@ client_with_mqueue(ClientId, Topic, Count) ->
     {ok, Client} = emqtt:start_link([
         {proto_ver, v5},
         {clientid, ClientId},
-        {clean_start, false},
+        {clean_start, true},
         {properties, #{'Session-Expiry-Interval' => 120}}
     ]),
     {ok, _} = emqtt:connect(Client),
     {ok, _, _} = emqtt:subscribe(Client, Topic, 1),
+    ct:sleep(300),
     ok = emqtt:disconnect(Client),
+    ct:sleep(100),
     publish_msgs(Topic, Count),
-    {ok, Client}.
+    ok.
 
 client_with_inflight(ClientId, Topic, Count) ->
     {ok, Client} = emqtt:start_link([
@@ -1275,13 +1277,18 @@ publish_msgs(Topic, Count) ->
 
 test_messages(Path, Topic, Count, AuthHeader, PayloadEncoding, IsMqueue) ->
     Qs0 = io_lib:format("payload=~s", [PayloadEncoding]),
-    {ok, MsgsResp} = emqx_mgmt_api_test_util:request_api(get, Path, Qs0, AuthHeader),
-    #{<<"meta">> := Meta, <<"data">> := Msgs} = emqx_utils_json:decode(MsgsResp),
-    #{<<"start">> := StartPos, <<"position">> := Pos} = Meta,
 
-    ?assertEqual(StartPos, msg_pos(hd(Msgs), IsMqueue)),
-    ?assertEqual(Pos, msg_pos(lists:last(Msgs), IsMqueue)),
-    ?assertEqual(length(Msgs), Count),
+    {Msgs, StartPos, Pos} = ?retry(500, 10, begin
+        {ok, MsgsResp} = emqx_mgmt_api_test_util:request_api(get, Path, Qs0, AuthHeader),
+        #{<<"meta">> := Meta, <<"data">> := Msgs} = emqx_utils_json:decode(MsgsResp),
+        #{<<"start">> := StartPos, <<"position">> := Pos} = Meta,
+
+        ?assertEqual(StartPos, msg_pos(hd(Msgs), IsMqueue)),
+        ?assertEqual(Pos, msg_pos(lists:last(Msgs), IsMqueue)),
+        ?assertEqual(length(Msgs), Count),
+
+        {Msgs, StartPos, Pos}
+    end),
 
     lists:foreach(
         fun({Seq, #{<<"payload">> := P} = M}) ->