Add specs for Thread#group #1735

Job	Run time
RuboCop	25s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 31s
specs (ubuntu, 3.2.2)	1m 25s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 30s
specs (macos, 3.1.4)	1m 25s
specs (macos, 3.2.2)	1m 26s
specs (macos, 3.3.0)	1m 14s
specs (windows, 3.0.6)	5m 6s
specs (windows, 3.1.4)	5m 4s
specs (windows, 3.2.2)	5m 8s
specs (windows, 3.3.0)	4m 46s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 27s
	33m 20s

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