tests/fate-run.sh: Don't overlook errors from md5 tests

tests/fate-run.sh: Don't overlook errors from md5 tests

The md5 test up until now ignored errors from ffmpeg (the cli) and just
md5'ed whatever ffmpeg has output; while testing scenarios in which
ffmpeg fails has its merits, errors should not be overlooked by default;
doing so also reduces the effectiveness of sanitizers as errors from
them are ignored. This has happened with a memleak in the AV1 decoder.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>