tests: Fix dead_code warning for tuple struct #2459

Job	Run time
cargo +stable build	42s
cargo +beta build	43s
cargo hack check --feature-powerset	2m 23s
cargo +nightly build	41s
cargo bench	50s
cargo +1.36 build (futures-{core, io, sink})	35s
cargo miri test	8m 54s
cargo build --target thumbv7m-none-eabi	26s
cargo test -Z sanitizer=address	5m 30s
cargo build --target thumbv6m-none-eabi	24s
cargo test (ubuntu-latest)	3m 2s
cargo test -Z sanitizer=memory	3m 58s
cargo test (macos-latest)	5m 26s
cargo +1.56 build	1m 12s
cargo test -Z sanitizer=thread	2m 55s
cargo test (windows-latest)	4m 50s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 10s
cargo clippy	25s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 46s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 48s
cargo doc	21s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 25s
cargo fmt	12s
cargo minimal-versions build	28s
	1h 4m 6s

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