Fix effect-free warning in ob-screen.el while loop

Fix effect-free warning in ob-screen.el while loop

The warning was introduced and detected by an optimizer addition
proposed in the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-12/msg00711.html

* lisp/org/ob-screen.el (org-babel-screen-test): Avoid 'value
returned from (format "...") is unused' warning by doing something
more useful than busy string manipulation while waiting for an
asynchronous subprocess to make the temporary file readable.