Add spec for `Socket#read_nonblock` with a provided buffer #200

Job	Run time
RuboCop	24s
specs (ubuntu, 3.0.6)	1m 29s
specs (ubuntu, 3.1.4)	1m 26s
specs (ubuntu, 3.2.2)	1m 26s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 48s
specs (macos, 3.1.4)	2m 10s
specs (macos, 3.2.2)	3m 22s
specs (macos, 3.3.0)	1m 49s
specs (windows, 3.0.6)	3m 16s
specs (windows, 3.1.4)	3m 47s
specs (windows, 3.2.2)	4m 17s
specs (windows, 3.3.0)	3m 42s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	31m 41s

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