add new benchmark for `FutureGroup` #266

Job	Run time
Build and test (miri, nightly)	1m 20s
Build and test (ubuntu-latest, stable)	1m 4s
Checking clippy, fmt and docs	33s
Build and test (windows-latest, stable)	2m 31s
Build and test (macOS-latest, stable)	1m 27s
	6m 55s

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