Behat: Test third level nav (arrow-bar) for groups

[Rebecca Blundell]

We don't currently test the third level nav for groups. This could be added to social/group-main.feature. (Note, this bug should be completed first: https://bugs.launchpad.net/mahara/+bug/1892119)

What to test:
1. Follow one of the groups
2. Check we are on the 'About' tab (note that 'bt-groupinfo' elements are checked in create_and_delete_groups.feature, so pick something else!)
3. Click each of the other 'Arrow-bar nav' buttons and check for one element that is unique to each of those pages. There is no need to add an example of each of the elements. E.g. for 'Journals', checking "There are no journals in this group" is displayed is a valid check for this test. We are checking that the nav bar elements are present and work, nothing further.