(with-current-buffer (get-buffer-create "*test buffer*")
  (let* ((oldproc (get-buffer-process (current-buffer))))
    (when oldproc
      (kill-process oldproc)
      (accept-process-output oldproc)))
  (erase-buffer)
  (display-buffer (current-buffer))
  (let* ((print-level nil)
         (print-length nil)
         (proc (start-process
                "test proc" (current-buffer)
                (concat invocation-directory invocation-name)
                "-Q" "--batch" "--eval"
                (prin1-to-string
                 '(let (s)
                    (while (setq s (read-from-minibuffer "$ "))
                      (princ s)
                      (princ "\n")))))))
    (send-string proc "one\n")
    (sit-for 0.5)                       ; Read "one".
    (set-process-filter proc t)         ; Stop reading from proc.
    (send-string proc "two\n")
    (sit-for 1)                         ; Can't read "two" yet.
    (set-process-filter
     proc                               ; Resume reading from proc.
     #'internal-default-process-filter)
    (sit-for 0.5)                       ; Read "two" from proc.
    ))