Function check_seekable_capability_honesty 

pub fn check_seekable_capability_honesty<D: DecoderConfig>(
    dec: D,
    data: &[u8],
) -> Conformance

Verify the seekable capability flag in both directions. A decoder declaring it must decode data through push_decoder_from_source over a slice source with the same pixels as the one-shot path; one not declaring it must reject with UnsupportedOperation::SeekableDecode.