Add specs for Random::Formatter#alphanumeric #1818

Job	Run time
RuboCop	32s
specs (ubuntu, 3.0.6)	1m 5s
specs (ubuntu, 3.1.4)	1m 28s
specs (ubuntu, 3.2.2)	1m 31s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 28s
specs (macos, 3.1.4)	1m 32s
specs (macos, 3.2.2)	1m 16s
specs (macos, 3.3.0)	1m 10s
specs (windows, 3.0.6)	4m 5s
specs (windows, 3.1.4)	4m 38s
specs (windows, 3.2.2)	4m 38s
specs (windows, 3.3.0)	4m 3s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	30m 14s

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