emacs -Q -l cl-lib \
  --eval "(let* (flag \
                 (proc (apply (function start-process) \"test\" nil \"sleep\" (list \"3\")))) \
            (set-process-sentinel proc (lambda (&rest _args) (setq flag t))) \
            (cl-assert (cl-loop repeat 10 \
                        until flag \
                        do (accept-process-output nil 0.5) \
                        finally return flag)) \
            (message \"good!\"))"