Choose FK name correctly during partition attachment

Choose FK name correctly during partition attachment

During ALTER TABLE ATTACH PARTITION, if the name of a parent's foreign
key constraint is already used on the partition, the code tries to
choose another one before the FK attributes list has been populated,
so the resulting constraint name was "__fkey" instead of
"__fkey".  Repair, and add a test case.

Backpatch to 12.  In 11, the code to attach a partition was not smart
enough to cope with conflicting constraint names, so the problem doesn't
exist there.

Author: Jehan-Guillaume de Rorthais 
Discussion: https://postgr.es/m/20220901184156.738ebee5@karst