Add specs for Method#/UnboundMethod#original_name through a three-ali… #2366

Job	Run time
RuboCop	45s
specs (ubuntu, 3.3.10)	1m 36s
specs (windows, 4.0.1)	3m 49s
specs (ubuntu, 4.0.1)	1m 36s
specs (ubuntu, 4.0.1, --repeat 2)	2m 26s
specs (ubuntu, 4.0.1, LC_ALL=C)	1m 54s
specs (ubuntu, 3.4.8)	1m 41s
specs (macos, 4.0.1)	1m 29s
specs (ubuntu, 4.0.1, --enable-frozen-string-literal)	1m 30s
specs (windows, 3.4.8)	3m 8s
specs (windows, 3.3.10)	3m 38s
specs (macos, 3.4.8)	1m 44s
specs (macos, 3.3.10)	1m 50s
specs (ubuntu, 4.0.1, --parser=parse.y)	1m 39s
	28m 45s

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