Add PostgreSQL support to test migrations.

This commit adds support for running the migrations tests
with a PostgreSQL backend.

Migration 012 was changed because PostgreSQL's constraint
drop uses a different name to identify the foreign key
to be removed. The migration now handles it properly for
both postgres and mysql.

Change-Id: Ie8850608fc5f4bccda57a0b12c3b4adc8d1f61d0
Merge "Add PostgreSQL support to test migrations."