Revise migration tests execution on master

It seems that the patch scripts are not applied on 7.9 and master for the migration tests.
For reference compare 7.8 and master pom.xml:

• https://github.com/camunda/camunda-bpm-platform-maintenance/blob/7.8/qa/test-db-instance-migration/test-fixture-78/pom.xml
• https://github.com/camunda/camunda-bpm-platform/tree/master/qa/test-db-instance-migration/test-fixture-78/pom.xml