[Form] fix ORMQueryBuilderLoader (Issue #7156)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 7156
| License       | MIT
| Doc PR        | -

Fixed bug occured e.g. on postgresql db, on sqlite everything was ok. I have no provide any new tests because this issue has been covered by tests already, but when sqlite db was used, tests passed. In first commit I added ability to run tests using different database driver.

To run tests using postgresql you should use command:

    phpunit -c phpunit-postgresql.xml.dist src/Symfony/Bridge/Doctrine/Tests

Default dsn: pgsql:dbname=symfony;user=postgres;password=1234

In order to ensure postgresql tests isolation I have provide transaction rollback teardown.