rabbit_feature_flags: Fix registry reset vs. Erlang embedded mode

[Why]
If the Erlang VM is running in embedded mode, it will refuse to
auto-load any module from disk. That's because all modules are loaded
from the Erlang release right from the start.

[How]
After purging the module, we explicitly reload the stub version from the
disk.

Author: dumbbell

deps/rabbit/src/rabbit_ff_registry_factory.erl

@@ -842,5 +842,9 @@ reset_registry() ->
        #{domain => ?RMQLOG_DOMAIN_FEAT_FLAGS}),
     _ = code:purge(rabbit_ff_registry),
     _ = code:delete(rabbit_ff_registry),
+    %% After purging the module, we explicitly reload the stub version from the
+    %% disk. We need to do this because if the Erlang VM is running in embedded
+    %% mode, it will refuse to auto-load the module.
+    _ = code:load_file(rabbit_ff_registry),
     ?assertNot(rabbit_ff_registry:is_registry_initialized()),
     ok.