rabbit_feature_flags: Export rabbit_ff_controller functions for internal use

The two exported functions are used by the testsuite. They should be
exported when `TEST` is defined only, but the way mixed-version testing
is implemented in Bazel forces us to export them.

Author: dumbbell

deps/rabbit/src/rabbit_ff_controller.erl

@@ -44,7 +44,9 @@
          start_link/0,
          rpc_call/5,
          all_nodes/0,
-         running_nodes/0]).
+         running_nodes/0,
+         collect_inventory_on_nodes/1, collect_inventory_on_nodes/2,
+         mark_as_enabled_on_nodes/4]).
 
 %% gen_statem callbacks.
 -export([callback_mode/0,