add new benchmark for `FutureGroup` #265

Job	Run time
Checking clippy, fmt and docs	30s
Build and test (ubuntu-latest, stable)	1m 1s
Build and test (miri, nightly)	1m 17s
Build and test (windows-latest, stable)	2m 9s
Build and test (macOS-latest, stable)	1m 33s
	6m 30s

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