pub(crate) fn rev(
        mut self,
        search: impl FnMut(&[u8], &[u8]) -> Option<Option<usize>> + 'static,
    ) -> Runner {
        self.rev = Some(Box::new(search));
        self
    }
Set the implementation for reverse 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 reverse substring search is not tested.
src/tests/substring/mod.rs