test: perform sanity checks when starting apps

These are checks to detect inter-suite or inter-testcase flakiness
early.  One suite might forget one application running
and stop others, and then the `application:start/2' callback is never
called again for this application.

One example of this was that: i) `emqx_rule_engine` was left running by
one suite; ii) `emqx` app was stopped, taking `emqx_config_handler`
down with it and losing the rule engine handler; iii) another suite
that uses rule engine "started" it (a no-op) and then the config
handler was never installed again.

apps/emqx/test/emqx_common_test_helpers.erl

@@ -251,6 +251,7 @@ start_app(App, SpecAppConfig, Opts) ->
         {ok, _} ->
             ok = ensure_dashboard_listeners_started(App),
             ok = wait_for_app_processes(App),
+            ok = perform_sanity_checks(App),
             ok;
         {error, Reason} ->
             error({failed_to_start_app, App, Reason})
@@ -264,6 +265,27 @@ wait_for_app_processes(emqx_conf) ->
 wait_for_app_processes(_) ->
     ok.
 
+%% These are checks to detect inter-suite or inter-testcase flakiness
+%% early.  For example, one suite might forget one application running
+%% and stop others, and then the `application:start/2' callback is
+%% never called again for this application.
+perform_sanity_checks(emqx_rule_engine) ->
+    ensure_config_handler(emqx_rule_engine, [rule_engine, rules]),
+    ok;
+perform_sanity_checks(emqx_bridge) ->
+    ensure_config_handler(emqx_bridge, [bridges]),
+    ok;
+perform_sanity_checks(_App) ->
+    ok.
+
+ensure_config_handler(Module, ConfigPath) ->
+    #{handlers := Handlers} = sys:get_state(emqx_config_handler),
+    case emqx_utils_maps:deep_get(ConfigPath, Handlers, not_found) of
+        #{{mod} := Module} -> ok;
+        _NotFound -> error({config_handler_missing, ConfigPath, Module})
+    end,
+    ok.
+
 app_conf_file(emqx_conf) -> "emqx.conf.all";
 app_conf_file(App) -> atom_to_list(App) ++ ".conf".