add new benchmark for `FutureGroup` #273

Job	Run time
msrv	33s
Build and test (ubuntu-latest, stable)	1m 11s
Build and test (miri, nightly)	1m 13s
Build and test (windows-latest, stable)	2m 21s
Checking clippy, fmt and docs	28s
semver-checks	14s
Build and test (macOS-latest, stable)	2m 56s
	8m 56s

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