test(pulsar_producer): attempt to stabilize flaky test

https://github.com/emqx/emqx/actions/runs/5125166433/jobs/9218613872?pr=10886#step:7:679
```
  =CRITICAL REPORT==== 30-May-2023::19:38:58.003170 ===
  Run stage failed: error:{badmatch,
                              {timeout,
                                  [#{msg => pulsar_producer_bridge_started,
                                     '~meta' =>
                                         #{gl => <97891.472.0>,
                                           location =>
                                               #Fun<emqx_bridge_pulsar_impl_producer.11.109752493>,
                                           node => 'autocluster_node1@127.0.0.1',
                                           pid => <97891.787.0>,
                                           time => -576460611692219}}]}}
  Stacktrace: [{emqx_bridge_pulsar_impl_producer_SUITE,'-t_cluster/1-fun-10-',
                   6,
                   [{file,
                        "/emqx/apps/emqx_bridge_pulsar/test/emqx_bridge_pulsar_impl_producer_SUITE.erl"},
                    {line,1073}]},
               {emqx_bridge_pulsar_impl_producer_SUITE,t_cluster,1,
                   [{file,
                        "/emqx/apps/emqx_bridge_pulsar/test/emqx_bridge_pulsar_impl_producer_SUITE.erl"},
                    {line,1064}]}]
```

apps/emqx/test/emqx_common_test_helpers.erl

@@ -756,8 +756,8 @@ start_slave(Name, Opts) when is_map(Opts) ->
             throw(Other)
     end,
     pong = net_adm:ping(Node),
-    setup_node(Node, Opts),
     ok = snabbkaffe:forward_trace(Node),
+    setup_node(Node, Opts),
     Node.
 
 %% Node stopping

apps/emqx_bridge/src/emqx_bridge_app.erl

@@ -17,6 +17,8 @@
 
 -behaviour(application).
 
+-include_lib("snabbkaffe/include/snabbkaffe.hrl").
+
 -export([start/2, stop/1]).
 
 -export([
@@ -34,6 +36,7 @@ start(_StartType, _StartArgs) ->
     ok = emqx_bridge:load_hook(),
     ok = emqx_config_handler:add_handler(?LEAF_NODE_HDLR_PATH, ?MODULE),
     ok = emqx_config_handler:add_handler(?TOP_LELVE_HDLR_PATH, emqx_bridge),
+    ?tp(emqx_bridge_app_started, #{}),
     {ok, Sup}.
 
 stop(_State) ->

apps/emqx_bridge_pulsar/test/emqx_bridge_pulsar_impl_producer_SUITE.erl

@@ -1064,9 +1064,17 @@ t_cluster(Config) ->
     ?check_trace(
         begin
             Nodes = [N1, N2 | _] = start_cluster(Cluster),
+            %% wait until bridge app supervisor is up; by that point,
+            %% `emqx_config_handler:add_handler' has been called and the node should be
+            %% ready to create bridges.
+            NumNodes = length(Nodes),
+            {ok, _} = snabbkaffe:block_until(
+                ?match_n_events(NumNodes, #{?snk_kind := emqx_bridge_app_started}),
+                15_000
+            ),
             {ok, SRef0} = snabbkaffe:subscribe(
                 ?match_event(#{?snk_kind := pulsar_producer_bridge_started}),
-                length(Nodes),
+                NumNodes,
                 15_000
             ),
             {ok, _} = erpc:call(N1, fun() -> create_bridge(Config) end),