Improve specs of SecureRandom.random_number #1825

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 30s
specs (ubuntu, 3.2.2)	1m 28s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 38s
specs (macos, 3.1.4)	1m 26s
specs (macos, 3.2.2)	1m 21s
specs (macos, 3.3.0)	1m 30s
specs (windows, 3.0.6)	4m 13s
specs (windows, 3.1.4)	7m 1s
specs (windows, 3.2.2)	5m 2s
specs (windows, 3.3.0)	4m 53s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 26s
	34m 52s

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