test(lib): fix unused warnings due to feature gating test imports #3128

Job	Run time
Verify hyper.h is up to date	31s
semver	22s
Check Style	14s
Test C API (FFI)	45s
features	39s
Check MSRV	5m 20s
Test with Miri	2m 15s
minimal-versions	34s
Test cargo-c support (FFI)	26s
udeps	21s
Test stable on windows-latest	2m 31s
Test beta on ubuntu-latest	1m 6s
Test beta on macOS-latest	2m 42s
Test beta on windows-latest	2m 7s
Test stable on ubuntu-latest	1m 10s
Test stable on macOS-latest	1m 52s
Test nightly on windows-latest	3m 3s
Test nightly on macOS-latest	1m 55s
Test nightly on ubuntu-latest	1m 8s
Check exposed types	23s
Build docs	20s
CI is green	3s
	29m 47s

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