bug#59317: 29.0.50; Feature idea: suppress `message' output in ert batch test output

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

>> A good number of Emacs tests exercise parts of Emacs that call
>> `message'.
>
> Are you sure?  Many Emacs features display text that doesn't go
> through 'message'.
>
>> Idea: hide `message' output when running in batch mode, printing them
>> only for failed tests.
>
> We have set-message-function that could be used for this purpose, I
> think?  If indeed 'message' is the culprit.

Is there any way to capture and stop *all* output in the terminal for
passing tests?  Like `set-batch-output-function', or something to that
effect.