Once we reintroduce the `shuffleMergeEnabled` method, all changes to this file (DAGSchedulerSuite) related to mergerLocs.isEmpty == !shuffleMergeEnabled (and reverse) can be reverted (for the latest [diff](https://github.com/apache/spark/pull/34122/commits/3b52bed7ea4bbdb127a4711201d8c8268ab06650#))  