I have a unit test I'm making for a mode I'm working on that kills a particular region and puts it in a variable but the test doesn't seem to register region-active-p as true

(ert-deftest kill-region-test ()
  (with-current-buffer (get-buffer-create "*rack em tests*")
    (rack-em-start)
    (insert "Testing\n123")
    (goto-char 0)
    (set-mark 0)
    (push-mark 4)
    (activate-mark)
    (should (equal nil rack-em--yank-list))
    (call-interactively 'rack-em-kill-region)
    (should (equal '("Test") rack-em--yank-list))))

all the rack-em-start does is initialize rack-em--yank-list as '()

inside rack-em-kill-region I have

(defun rack-em-kill-region
  (beg end)
  (interactive (list
                (region-beginning)
                (region-end)))
  (if (region-active-p)
      (progn
        (kill-region beg end)
        (rack-em--add-from-kill-ring))))

and I can't seem to debug from the overseer test in emacs either

(defun rack-em--add-from-kill-ring ()
  (add-to-list 'rack-em--yank-list
               (substring-no-properties (first kill-ring))))