seek-stress: Only run on IVF files
Currently seek-stress is only implemented for IVF files. On section5 or annexb files an error message from the ivf demuxer is printed which includes the first 4 bytes of the file in ASCII and hex representation. The section5 example starts with 0x12 which is an ASCII control character, leading to problems in the JUnit XML file automatically generated by meson.
Related: dav1d!1132 (merged)