Add specs for MatchData#bytebegin, #byteend #2107

Job	Run time
RuboCop	56s
specs (ubuntu, 3.3.9)	1m 31s
specs (windows, 3.4.7)	3m 24s
specs (ubuntu, 3.4.7)	1m 39s
specs (ubuntu, 3.4.7, --enable-frozen-string-literal)	1m 41s
specs (macos, 3.3.9)	1m 13s
specs (macos, 3.4.7)	1m 52s
specs (ubuntu, 3.4.7, --parser=parse.y)	1m 40s
specs (windows, 3.3.9)	4m 3s
specs (macos, 3.2.9)	2m 7s
specs (windows, 3.2.8)	3m 28s
specs (ubuntu, 3.2.9)	1m 36s
	25m 10s

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