refac(axum-extra): improve test invariants for `protobuf.rs` extractor #5883

Job	Run time
check	40s
check-docs	26s
cargo-hack	2m 47s
cargo-public-api-crates (axum-core)	37s
cargo-deny check (bans licenses sources)	27s
cargo-public-api-crates (axum)	57s
cargo-public-api-crates (axum-macros)	40s
cargo-deny check (advisories)	22s
cargo-public-api-crates (axum-extra)	53s
Spell Check with Typos	6s
dependencies-are-sorted	6s
test-versions	0s
wasm32-unknown-unknown	0s
test-docs	0s
test-nightly	0s
armv5te-unknown-linux-musleabi	0s
test-msrv	0s
	8m 1s

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