Per-discussion subscriptions are maintained after backup and restore

This test requires administrator access and five student accounts.

1. Create a forum and in the Subscription and tracking section, select 'Optional' from the subscription mode.
2. Create two discussions in this forum.
3. As student1, subscribe to discussion 1 by clicking the toggle switch under the subscribe column
   
4. As student2,subscribe to discussion 2
5. As student3,subscribe to the forum
6. As student4,subscribe to the forum and unsubscribe from discussion 1
7. As student5, subscribe to the forum and unsubscribe from discussion 2
8. As admin, backup the forum including user data
9. Restore the forum.
10. For each of the users above, confirm that the subscription data was copied over correctly