test-ol/org-toggle-link-display: Fix compatibility with old Emacs

* testing/lisp/test-ol.el (test-ol/org-toggle-link-display): Use
back-compatible `org-xor' instead of `xor'.

testing/lisp/test-ol.el

@@ -59,19 +59,19 @@ See https://github.com/yantar92/org/issues/4."
       (dotimes (_ 2)
         (goto-char 1)
         (re-search-forward "\\[")
-        (should-not (xor org-link-descriptive (org-invisible-p)))
+        (should-not (org-xor org-link-descriptive (org-invisible-p)))
         (re-search-forward "example")
-        (should-not (xor org-link-descriptive (org-invisible-p)))
+        (should-not (org-xor org-link-descriptive (org-invisible-p)))
         (re-search-forward "com")
-        (should-not (xor org-link-descriptive (org-invisible-p)))
+        (should-not (org-xor org-link-descriptive (org-invisible-p)))
         (re-search-forward "]")
-        (should-not (xor org-link-descriptive (org-invisible-p)))
+        (should-not (org-xor org-link-descriptive (org-invisible-p)))
         (re-search-forward "\\[")
         (should-not (org-invisible-p))
         (re-search-forward "link")
         (should-not (org-invisible-p))
         (re-search-forward "]")
-        (should-not (xor org-link-descriptive (org-invisible-p)))
+        (should-not (org-xor org-link-descriptive (org-invisible-p)))
         (org-toggle-link-display)))))