We're currently maintaining two outputs for cube regression test.  But that
appears to be unsuitable, because these outputs are different in out few checks
involving scientific notation.  So, split checks involving scientific notation
into separate test, making contrib/cube easier to maintain.  Backpatch to all
supported versions in order to make further backpatching easier.

Discussion: https://postgr.es/m/CAPpHfdvJgWjxHsJTtT%2Bo1tz3OR8EFHcLQjhp-d3%2BUcmJLh-fQA%40mail.gmail.com
Author: Alexander Korotkov
Backpatch-through: 9.3