Undo the change in test_if_skip_sort_order() that set ref_key=-1 when
a variant of index_merge is used (was made in fix for MDEV-9021).

It turned out that test_if_cheaper_ordering() call below assumes that
ref_key=-1 means "no index is used", that is, "an inefficient full table
scan is done".
This is not the same as index_merge, index_merge can actually be quite
efficient. So, ref_key=MAX_KEY denotes the fact that some index is used,
not any given index.