Add specs for lookarounds in regexp in linear time #1823

Job	Run time
RuboCop	35s
specs (ubuntu, 3.0.6)	1m 32s
specs (ubuntu, 3.1.4)	1m 31s
specs (ubuntu, 3.2.2)	1m 24s
specs (ubuntu, 3.3.0)	1m 29s
specs (macos, 3.0.6)	1m 41s
specs (macos, 3.1.4)	1m 42s
specs (macos, 3.2.2)	1m 21s
specs (macos, 3.3.0)	1m 18s
specs (windows, 3.0.6)	4m 38s
specs (windows, 3.1.4)	5m 36s
specs (windows, 3.2.2)	4m 45s
specs (windows, 3.3.0)	4m 47s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	33m 44s

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