Add specs for Thread#group #1734

Job	Run time
RuboCop	24s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 31s
specs (ubuntu, 3.2.2)	1m 29s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 43s
specs (macos, 3.1.4)	1m 27s
specs (macos, 3.2.2)	1m 13s
specs (macos, 3.3.0)	1m 24s
specs (windows, 3.0.6)	4m 43s
specs (windows, 3.1.4)	5m 25s
specs (windows, 3.2.2)	3m 51s
specs (windows, 3.3.0)	3m 42s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 26s
	31m 8s

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