'Delete and then restore' doesn't delete when using asynchronous restore

Additional unit tests provided. Prerequisites: Enable the ability to asynchronously backup (and restore) courses. Use the instructions  here Create a course with an assignment and forum taking default values. Back up the course with default settings following instructions here Download the backup file when completed. Instructions: As an admin, go to the course created. Go to the restore page by following the instructions here Select the backup created in Prerequisites#3 OR upload the downloaded file from Prerequisites#4 During the restore sequence, on the 'Destination' step, choose to 'Delete the contents of this course and then restore' Proceed through the rest of the steps taking the defaults until finally the restore operation has been triggered Once the restore has completed, return to the restored course Confirm the original content in the destination course is deleted and the content from the restore file is added Repeat the test with synchronous backup/restore by resetting Prerequisites#1 back to disabled.