pub(crate) fn fwd(
        mut self,
        search: impl FnMut(&[u8], &[u8], u8, u8) -> Option<Option<usize>> + 'static,
    ) -> Runner {
        self.fwd = Some(Box::new(search));
        self
    }
Set the implementation for forward "packed pair" substring search.

If the closure returns `None`, then it is assumed that the given
test cannot be applied to the particular implementation and it is
skipped. For example, if a particular implementation only supports
needles or haystacks for some minimum length.

If this is not set, then forward "packed pair" search is not tested.
src/tests/packedpair.rs