avutil: check pixdescs in a different place

Doing this check in avutil_version() is not appropriate. Also, this code
is by default disabled (--assert-level is by default 0). A FATE run with
defaults will never execute the checks.

Move it to the pixelutils test program. Whatever reason there was in
avutil_version() not to run this test by default, it should be fine in
this test program. This means FATE will run the test by default. (Yes,
pixelutils is not strictly the best place for it either, but it's
better.)

(pixdesc.c also has a small test program, but it's never run by FATE.)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
