rabbit_prelaunch_early_logging: Always export APIs needed in tests

[Why]
Before `rabbitmq_prelaunch` was moved from `deps/rabbit/apps` to `deps`,
it would inherit compile flags from `deps/rabbit`. Therefore, when
`rabbit` was tested, `rabbitmq_prelaunch` would be compiled with
`-DTEST`.

After the move, this is not the case anymore. Thus, the `logging_SUITE`
testsuite can't call those internal functions anymore.

[How]
Now, the needed functions are always exported. We simply comment that
they are for internal testing purpose and should not be called in
production code.

Author: dumbbell

deps/rabbitmq_prelaunch/src/rabbit_prelaunch_early_logging.erl

@@ -25,10 +25,9 @@
 -export([filter_log_event/2]).
 -export([filter_discarded_message/2]).
 
--ifdef(TEST).
+%% For internal testing purpose only.
 -export([levels/0,
          determine_prefix/1]).
--endif.
 
 -define(CONFIGURED_KEY, {?MODULE, configured}).