Additional shared spec for StringIO#sysread #296

Job	Run time
RuboCop	31s
specs (ubuntu, 3.0.6)	1m 28s
specs (ubuntu, 3.1.4)	1m 33s
specs (ubuntu, 3.2.2)	1m 28s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 24s
specs (macos, 3.1.4)	1m 21s
specs (macos, 3.2.2)	1m 12s
specs (macos, 3.3.0)	1m 9s
specs (windows, 3.0.6)	5m 9s
specs (windows, 3.1.4)	5m 29s
specs (windows, 3.2.2)	5m 25s
specs (windows, 3.3.0)	5m 10s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 21s
	34m 3s

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