transactional migration test-case was broken.

The cleanup commit a85625d broke the test-case.
The schema was no longer modified so there was no
way to check that the rollback actually happened.
transactions can be turned off per Migration.

Closes #9483.

There are SQL Queries that can't run inside a transaction. Since
the Migrator used to wrap all Migrations inside a transaction there
was no way to run these queries within a migration.

This patch adds `self.disable_ddl_transaction!` to the migration to
turn transactions off when necessary.
Merge pull request #9507 from senny/9483_migrations_without_transactions

Transactions can be turned off per Migration