Reenable sleep ns spec on MacOS #1798

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 32s
specs (ubuntu, 3.2.2)	1m 24s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	3m 44s
specs (macos, 3.1.4)	3m 15s
specs (macos, 3.2.2)	2m 35s
specs (macos, 3.3.0)	3m 10s
specs (windows, 3.0.6)	4m 33s
specs (windows, 3.1.4)	4m 56s
specs (windows, 3.2.2)	4m 39s
specs (windows, 3.3.0)	4m 32s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	39m 5s

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