The last pixel format gets leaked as `$test` further down the pipeline.

See for example https://fate.libav.org/x86_32-netbsd-clang-no-inline-asm/20150420020104
Note the odd test names like “yuvj444p.”

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>