Fix WebP reference test #1771

Job	Run time
test_toolchains (1.70.0)	41s
test_other_archs (powerpc-unknown-linux-gnu)	2m 32s
test_toolchains (nightly)	1m 7s
build_benchmarks	42s
test_toolchains (beta)	1m 20s
Fuzz targets (afl)	2m 13s
test_other_archs (i686-unknown-linux-gnu)	1m 18s
Fuzz targets (cargo-fuzz)	50s
cargo-deny	15s
clippy	2m 14s
public_private_dependencies	24s
rustfmt	13s
test_avif_decoding	1m 32s
test_features	21s
cargo-semver-checks	29s
test_features (default)	1m 46s
test_features (rayon)	25s
test_features (avif)	1m 2s
test_features (bmp)	28s
test_features (dds)	28s
test_features (exr)	40s
test_features (ff)	30s
test_features (gif)	24s
test_features (hdr)	26s
test_features (ico)	32s
test_features (jpeg)	23s
test_features (png)	27s
test_features (pnm)	22s
test_features (qoi)	27s
test_features (tga)	22s
test_features (tiff)	27s
test_features (webp)	29s
	25m 49s

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