Reenable sleep ns spec on MacOS #1796

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	27s
specs (ubuntu, 3.1.4)	27s
specs (ubuntu, 3.2.2)	23s
specs (ubuntu, 3.3.0)	22s
specs (macos, 3.0.6)	3m 34s
specs (macos, 3.1.4)	3m 34s
specs (macos, 3.2.2)	3m 5s
specs (macos, 3.3.0)	2m 54s
specs (windows, 3.0.6)	1m 54s
specs (windows, 3.1.4)	3m 3s
specs (windows, 3.2.2)	2m 52s
specs (windows, 3.3.0)	3m 34s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	23s
	27m 2s

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