Fixed 2 problems :
1. test_if_order_by_key() was continuing on the primary key
as if it has a primary key suffix (as the secondary keys do).
This leads to crashes in ORDER BY <pk>,<pk>.
Fixed by not treating the primary key as the secondary one
and not depending on it being clustered with a primary key.
2. The cost calculation was trying to read the records 
per key when operating on ORDER BYs that order on all of the 
secondary key + some of the primary key.
This leads to crashes because of out-of-bounds array access.
Fixed by assuming we'll find 1 record per key in such cases.