This patch adds test cases to detect edge cases when finding
keylist columns.

The patch originated during development of D12052 to make sure
the new implementation matches the old implementation. It would
be good to add these test cases to master so this part is covered
in a next change might influence the expected edges.

The patch covers ED_keylist_find_next, ED_keylist_find_prev
and ED_keylist_find_exact methods.