In ert.el of Emacs 27.1, we have a simple ability to override the 'message'
function when running ert tests interactively by setting the message-fn
argument here:

;; Should OUTPUT-BUFFER-NAME and MESSAGE-FN really be arguments here?
;; They are needed only for our automated self-tests at the moment.
;; Or should there be some other mechanism?
;;;###autoload
(defun ert-run-tests-interactively (selector
                                    &optional output-buffer-name message-fn)

But in recent Emacs versions this function takes only the selector
argument, with no clear way of how to override messaging behavior so that a
message from a test case (rather than ert itself) can remain in the
minibuffer after a test case finishes:

(defun ert-run-tests-interactively (selector)
  "Run the tests specified by SELECTOR and display the results in a buffer.

SELECTOR works as described in `ert-select-tests'."

This would be very useful behavior to have back.  Is there any simple way
to do it now or can you add it back in?

Thanks,

Bob