Specs for StringIO#sysread and StringIO#read_nonblock #1764

Job	Run time
RuboCop	29s
specs (ubuntu, 3.0.6)	1m 38s
specs (ubuntu, 3.1.4)	1m 32s
specs (ubuntu, 3.2.2)	1m 31s
specs (ubuntu, 3.3.0)	1m 32s
specs (macos, 3.0.6)	1m 23s
specs (macos, 3.1.4)	1m 25s
specs (macos, 3.2.2)	1m 8s
specs (macos, 3.3.0)	1m 12s
specs (windows, 3.0.6)	6m 10s
specs (windows, 3.1.4)	5m 22s
specs (windows, 3.2.2)	5m 19s
specs (windows, 3.3.0)	5m 18s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 27s
	35m 26s

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