Improve specs for Time#gmtime/Time#utc #1778

Job	Run time
RuboCop	32s
specs (ubuntu, 3.0.6)	1m 32s
specs (ubuntu, 3.1.4)	1m 36s
specs (ubuntu, 3.2.2)	1m 23s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 23s
specs (macos, 3.1.4)	1m 25s
specs (macos, 3.2.2)	1m 13s
specs (macos, 3.3.0)	1m 9s
specs (windows, 3.0.6)	4m 26s
specs (windows, 3.1.4)	4m 47s
specs (windows, 3.2.2)	4m 44s
specs (windows, 3.3.0)	3m 40s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 24s
	30m 37s

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