fate: add simple test for async

This only checks that things havnt changed, the values provide little
help in determining if a change is good or bad.

Improvements welcome!

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
tests: Replace fieldmatch test with pixfmt

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>