Add specs for Random::Formatter#alphanumeric #1828

Job	Run time
RuboCop	35s
specs (ubuntu, 3.0.6)	1m 38s
specs (ubuntu, 3.1.4)	1m 35s
specs (ubuntu, 3.2.2)	1m 29s
specs (ubuntu, 3.3.0)	1m 27s
specs (macos, 3.0.6)	1m 26s
specs (macos, 3.1.4)	1m 29s
specs (macos, 3.2.2)	1m 9s
specs (macos, 3.3.0)	1m 27s
specs (windows, 3.0.6)	3m 20s
specs (windows, 3.1.4)	4m 50s
specs (windows, 3.2.2)	5m 47s
specs (windows, 3.3.0)	3m 52s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 28s
	31m 32s

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