Improve specs of SecureRandom.random_number #330

Job	Run time
RuboCop	32s
specs (ubuntu, 3.0.6)	1m 35s
specs (ubuntu, 3.1.4)	1m 34s
specs (ubuntu, 3.2.2)	1m 28s
specs (ubuntu, 3.3.0)	1m 26s
specs (macos, 3.0.6)	1m 32s
specs (macos, 3.1.4)	1m 41s
specs (macos, 3.2.2)	1m 42s
specs (macos, 3.3.0)	1m 24s
specs (windows, 3.0.6)	3m 40s
specs (windows, 3.1.4)	4m 56s
specs (windows, 3.2.2)	4m 3s
specs (windows, 3.3.0)	4m 39s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	31m 35s

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