Add more tests for reopening classes and modules #2206

Job	Run time
RuboCop	52s
specs (ubuntu, 3.4.7)	1m 34s
specs (macos, 3.3.9)	1m 16s
specs (windows, 3.4.7)	3m 17s
specs (ubuntu, 3.2.9)	1m 31s
specs (macos, 3.2.9)	1m 52s
specs (macos, 3.4.7)	1m 17s
specs (ubuntu, 3.4.7, --enable-frozen-string-literal)	1m 39s
specs (windows, 3.3.9)	3m 5s
specs (ubuntu, 3.3.9)	1m 28s
specs (windows, 3.2.8)	6m 6s
specs (ubuntu, 3.4.7, --repeat 2)	2m 27s
specs (ubuntu, 3.4.7, --parser=parse.y)	1m 42s
	28m 6s

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