Add spec for `Socket#read_nonblock` with a provided buffer #1581

Job	Run time
RuboCop	26s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 33s
specs (ubuntu, 3.2.2)	1m 24s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 39s
specs (macos, 3.1.4)	1m 39s
specs (macos, 3.2.2)	1m 41s
specs (macos, 3.3.0)	1m 51s
specs (windows, 3.0.6)	3m 28s
specs (windows, 3.1.4)	3m 28s
specs (windows, 3.2.2)	4m 59s
specs (windows, 3.3.0)	5m 7s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 23s
	31m 31s

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