Use `IntoFuture` for test `RequestBuilder` #4238

Job	Run time
check-docs	25s
cargo-hack	3m 10s
check	38s
cargo-public-api-crates (axum)	30s
cargo-deny check (advisories)	28s
cargo-public-api-crates (axum-core)	25s
dependencies-are-sorted	18s
cargo-deny check (bans licenses sources)	27s
Spell Check with Typos	5s
cargo-public-api-crates (axum-extra)	28s
cargo-public-api-crates (axum-macros)	17s
test-nightly	1m 31s
test-versions (stable)	1m 57s
test-versions (beta)	1m 56s
test-docs	1m 37s
armv5te-unknown-linux-musleabi	31s
test-msrv	2m 12s
wasm32-unknown-unknown	25s
	17m 20s

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