Improve checks for actual duration in sleep specs #312

Job	Run time
RuboCop	42s
specs (ubuntu, 3.0.6)	1m 29s
specs (ubuntu, 3.1.4)	1m 29s
specs (ubuntu, 3.2.2)	1m 25s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	2m 3s
specs (macos, 3.1.4)	1m 41s
specs (macos, 3.2.2)	1m 36s
specs (macos, 3.3.0)	1m 15s
specs (windows, 3.0.6)	4m 43s
specs (windows, 3.1.4)	5m 26s
specs (windows, 3.2.2)	4m 52s
specs (windows, 3.3.0)	4m 47s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	34m 14s

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