rabbit_feature_flags: Add testcase after issue #12963

[Why]
Up-to RabbitMQ 3.13.x, there was a case where if:
1. you enabled a plugin
2. you enabled its feature flags
3. you disabled the plugin
4. you restarted a node (or upgraded it)

... the node could crash on startup because it had a feature flag marked
as enabled that it didn't know about:

    error:{badmatch,#{feature_flags => ...

        rabbit_ff_controller:-check_one_way_compatibility/2-fun-0-/3, line 514
        lists:all_1/2, line 1520
        rabbit_ff_controller:are_compatible/2, line 496
        rabbit_ff_controller:check_node_compatibility_task1/4, line 437
        rabbit_db_cluster:check_compatibility/1, line 376

This was "fixed" by the new way of keeping the registry in memory
(#10988) because it introduces a slight change of behavior. Indeed, the
old way walked through the `FeatureFlags` map and looked up the state in
the `FeatureStates` map to create the `is_enabled/1` function. The new
way just looks up the state in `FeatureStates`.

[How]
The new testcase succeeds on 4.0.x and `main`, but would fail on 3.13.x
with the aforementionne crash.

Author: dumbbell

deps/rabbit/test/feature_flags_SUITE.erl

@@ -46,7 +46,8 @@
          clustering_ok_with_supported_required_ff/1,
          activating_plugin_with_new_ff_disabled/1,
          activating_plugin_with_new_ff_enabled/1,
-         enable_plugin_feature_flag_after_deactivating_plugin/1
+         enable_plugin_feature_flag_after_deactivating_plugin/1,
+         restart_node_with_unknown_enabled_feature_flag/1
         ]).
 
 suite() ->
@@ -97,7 +98,8 @@ groups() ->
       [
        activating_plugin_with_new_ff_disabled,
        activating_plugin_with_new_ff_enabled,
-       enable_plugin_feature_flag_after_deactivating_plugin
+       enable_plugin_feature_flag_after_deactivating_plugin,
+       restart_node_with_unknown_enabled_feature_flag
       ]}
     ],
 
@@ -1292,6 +1294,53 @@ enable_plugin_feature_flag_after_deactivating_plugin(Config) ->
     end,
     ok.
 
+restart_node_with_unknown_enabled_feature_flag(Config) ->
+    FFSubsysOk = is_feature_flag_subsystem_available(Config),
+
+    log_feature_flags_of_all_nodes(Config),
+    case FFSubsysOk of
+        true ->
+            ?assertEqual([false, false],
+                         is_feature_flag_supported(Config, plugin_ff)),
+            ?assertEqual([false, false],
+                         is_feature_flag_enabled(Config, plugin_ff));
+        false ->
+            ok
+    end,
+
+    rabbit_ct_broker_helpers:enable_plugin(Config, 0, "my_plugin"),
+    rabbit_ct_broker_helpers:enable_plugin(Config, 1, "my_plugin"),
+    rabbit_ct_broker_helpers:disable_plugin(Config, 0, "my_plugin"),
+    rabbit_ct_broker_helpers:disable_plugin(Config, 1, "my_plugin"),
+
+    enable_feature_flag_on(Config, 0, plugin_ff),
+    case FFSubsysOk of
+        true ->
+            enable_feature_flag_on(Config, 0, plugin_ff),
+            ?assertEqual([true, true],
+                         is_feature_flag_supported(Config, plugin_ff)),
+            ?assertEqual([true, true],
+                         is_feature_flag_enabled(Config, plugin_ff));
+        false ->
+            ok
+    end,
+
+    rabbit_ct_broker_helpers:restart_node(Config, 0),
+    rabbit_ct_broker_helpers:restart_node(Config, 1),
+
+    log_feature_flags_of_all_nodes(Config),
+    case FFSubsysOk of
+        true ->
+            enable_feature_flag_on(Config, 0, plugin_ff),
+            ?assertEqual([false, false],
+                         is_feature_flag_supported(Config, plugin_ff)),
+            ?assertEqual([false, false],
+                         is_feature_flag_enabled(Config, plugin_ff));
+        false ->
+            ok
+    end,
+    ok.
+
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