test(maitake-sync): test futures impl `Future` #1455

Job	Run time
changed_paths	4s
clippy	2m 50s
Miri tests (cordyceps)	2m 58s
build boot image (x86_64)	3m 5s
Loom tests (cordyceps)	1m 26s
cargo check (host)	3m 40s
docs	4m 15s
Loom tests (maitake)	4m 24s
Miri tests (maitake)	33m 23s
Tests (maitake, no-default-features)	1m 31s
rustfmt	14s
cargo test (host)	4m 33s
cargo test (cross x64)	5m 51s
all_systems_go	1s
	1h 8m 15s

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