Reenable sleep ns spec on MacOS #1801

Job	Run time
RuboCop	33s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 31s
specs (ubuntu, 3.2.2)	1m 26s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 54s
specs (macos, 3.1.4)	1m 40s
specs (macos, 3.2.2)	1m 58s
specs (macos, 3.3.0)	1m 13s
specs (windows, 3.0.6)	3m 54s
specs (windows, 3.1.4)	4m 20s
specs (windows, 3.2.2)	5m 32s
specs (windows, 3.3.0)	3m 59s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	32m 18s

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