Reenable sleep ns spec on MacOS #1799

Job	Run time
RuboCop	34s
specs (ubuntu, 3.0.6)	1m 29s
specs (ubuntu, 3.1.4)	1m 30s
specs (ubuntu, 3.2.2)	1m 30s
specs (ubuntu, 3.3.0)	1m 28s
specs (macos, 3.0.6)	4m 16s
specs (macos, 3.1.4)	4m 3s
specs (macos, 3.2.2)	3m 36s
specs (macos, 3.3.0)	3m 11s
specs (windows, 3.0.6)	4m 6s
specs (windows, 3.1.4)	4m 3s
specs (windows, 3.2.2)	4m 3s
specs (windows, 3.3.0)	3m 58s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	39m 12s

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