Add "reads after ungetc with multibyte characters in the buffer" spec… #2243

Job	Run time
RuboCop	56s
specs (ubuntu, 3.2.9)	1m 34s
specs (ubuntu, 3.4.7, --repeat 2)	2m 26s
specs (ubuntu, 3.4.7)	1m 41s
specs (windows, 3.4.7)	3m 21s
specs (ubuntu, 3.3.9)	1m 29s
specs (macos, 3.4.7)	1m 22s
specs (macos, 3.2.9)	2m 6s
specs (macos, 3.3.9)	1m 18s
specs (windows, 3.2.8)	4m 14s
specs (windows, 3.3.9)	3m 33s
specs (ubuntu, 3.4.7, --enable-frozen-string-literal)	1m 38s
specs (ubuntu, 3.4.7, --parser=parse.y)	1m 38s
	27m 16s

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