The feedback from PlUnit is sent to the user_error stream. Now it depends a little on what you want to achieve. If this is a batch run from a shell script I’d just use the shell to redirect stderr. If not, things get a little nastier as there is no with_output_to/2 variant for user_error. You can bind it to another stream using
<create a stream Stream>
set_stream(Stream, alias(user_error))