Search starred discussions only option in forum advanced search

Behat tests provided

Prerequisites:

1. A site forum with a discussion in it with the content
   1. Message - "This is a site forum: SiteForum"
2. A course with a forum activity with 2 discussions with the following content:
   1. Discussion 1: Message - "This is discussion 1: Discussion1"
   2. Discussion 2: Message - "This is discussion 2: Discussion2"
3. As an admin, go to the forum activity and star Discussion 2 using the action menu OR by clicking the star icon on the left most column.

Testing Instructions:

1. As admin, go to the forum within the created course
2. Click on the search button on the top which redirects to the advanced search form
3. Confirm you see a checkbox at the bottom labelled "Search starred discussions only"
4. In the field "These words can appear anywhere in the post" enter the text 'discussion'
5. Click on the search button.
6. Confirm Discussion 1 and discussion 2 appears in the results
7. Go back to the Advanced search form.
8. Repeat #4 and #5.
9. Check the "Search starred discussions only" box.
10. Click on the search button.
11. Confirm only Discussion 2 appears in the results
12. Go back to the Advanced search form by clicking the 'Advanced search' link.
13. Confirm that the "Search starred discussions only" checkbox is checked.
14. Clear all the search criteria entered previously.
15. Repeat #9 - #11
16. Logout
17. Go to the site forum created and repeat #2
18. Confirm you do not see a checkbox at the bottom labelled "Search starred discussions only"