Move sleep ns precision spec to kernel and omit darwin #24

Job	Run time
RuboCop	33s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 27s
specs (ubuntu, 3.2.2)	1m 26s
specs (ubuntu, 3.3.0)	1m 29s
specs (macos, 3.0.6)	1m 32s
specs (macos, 3.1.4)	1m 23s
specs (macos, 3.2.2)	1m 15s
specs (macos, 3.3.0)	1m 13s
specs (windows, 3.0.6)	4m 39s
specs (windows, 3.1.4)	5m 2s
specs (windows, 3.2.2)	4m 58s
specs (windows, 3.3.0)	4m 22s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	32m 9s

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