The gap and overlap fixtures cut timestamps with aselect and asetpts.
ffmpeg 8 regroups the filter's audio into encoder frames and gives each
the timestamp of its first sample, so on ubuntu-latest the gap fixture
came out with no gap and the timeline test found the sine where the
seek expects silence. Those fixtures' source frames are now exactly
the encoder's (960 samples for Opus, 64 for Vorbis), which ffmpeg
hands on as they are, timestamps included.