A course reverts to the default format if the course format is deleted

This test requires a course set to topics format but with the default course for the site being Weekly format (as defined in Settings>Site administration>Courses>Course default formats)

Test steps:
1. As admin, navigate to Settings>Site administration>Plugins>Course formats>Manage course formats
2. Verify that the default course format is Weekly and can't be deleted.
3. Delete the Topics format.
4. As a teacher, go to the course and verify that it is now in the default Weekly format.