A key may be chosen in test_if_skip_sort_order(), after calling
test_if_cheaper_ordering(). The latter iterates over the keys, and
call test_if_order_by_key() to determine whether the key can be used
in ORDER BY.

We make it so that if the result of test_if_order_by_key() indicates
the current key under consideration would result in a better matching
ordering (e.g. ORDER BY DESC matches a desc key better than an asc
key), then we choose that key, given that the range_cost is the same.

TODO: consider whether it is possible to quantify the order matching
by passing direction to get_range_limit_read_cost.