Don't scale down time for overlay tests on MacOS

[pestctrl]

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.

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Thanks!

If you're just starting out to hack on CIDER you might find this section of its
manual extremely useful.

[ikappaki]

Hi, just to clarify, the macos tests have moved to circleci under ci/circle: test-macos-emacs-latest with #3278 and it is green now with this PR:

So I think this PR is good to go.

[bbatsov]

Thanks!