Don't scale down time for overlay tests on MacOS

Overlay tests involve testing the timer functionality of overlays, and
require the use of sleep. The tests scale all time-related arguments
by a certain factor to avoid overly inflating test-running
times.

The scaling factor was initially set at 1 order of magnitude slower
than the consistently succeeding value on my machine, but the value is
apparently still too fast for the MacOS test runner, so I have added
MacOS to the list of platforms unsafe to do this optimization for.

test/cider-overlay-tests.el

@@ -67,7 +67,7 @@ being set that way"
 (defun cider-overlay--safe-to-speed-up-tests ()
   (and (<= 28 emacs-major-version)
        (not (member system-type
-                    '(ms-dos windows-nt cygwin)))))
+                    '(ms-dos windows-nt cygwin darwin)))))
 
 (describe "cider--make-result-overlay"
   :var (overlay-count this-command)