Add parse.y to specs runner #1994

Job	Run time
RuboCop	52s
specs (ubuntu, 3.2.7)	1m 31s
specs (ubuntu, 3.3.7)	1m 26s
specs (ubuntu, 3.4.2)	1m 34s
specs (macos, 3.2.7)	1m 16s
specs (macos, 3.3.7)	1m 11s
specs (macos, 3.4.2)	1m 15s
specs (windows, 3.2.7)	3m 18s
specs (windows, 3.3.7)	2m 52s
specs (windows, 3.4.2)	3m 14s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 32s
specs (ubuntu, 3.4.2, --parser=parse.y)	1m 37s
	21m 38s

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