Improvements to specs for Random::Formatter #1845

Job	Run time
RuboCop	35s
specs (ubuntu, 3.0.6)	1m 36s
specs (ubuntu, 3.1.4)	1m 35s
specs (ubuntu, 3.2.2)	1m 29s
specs (ubuntu, 3.3.0)	1m 28s
specs (macos, 3.0.6)	1m 26s
specs (macos, 3.1.4)	1m 25s
specs (macos, 3.2.2)	1m 11s
specs (macos, 3.3.0)	1m 31s
specs (windows, 3.0.6)	4m 20s
specs (windows, 3.1.4)	4m 26s
specs (windows, 3.2.2)	5m 58s
specs (windows, 3.3.0)	5m 59s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 29s
	34m 28s

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