feat: expose test client under testing feature #4492

Job	Run time
check	42s
check-docs	31s
cargo-hack	26s
cargo-public-api-crates (axum)	29s
cargo-deny check (advisories)	23s
cargo-public-api-crates (axum-core)	19s
cargo-public-api-crates (axum-extra)	31s
dependencies-are-sorted	17s
cargo-deny check (bans licenses sources)	23s
cargo-public-api-crates (axum-macros)	16s
Spell Check with Typos	4s
test-msrv	0s
test-docs	0s
test-nightly	0s
armv5te-unknown-linux-musleabi	0s
test-versions	0s
wasm32-unknown-unknown	0s
	4m 21s

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