Use sets for comparison of accessible pages

Before
=====
```
>       assert menu.visible_pages() == group_data
E       assert ['about', 'ch...astores', ...] == ['about', 'cha...astores', ...]
E         At index 8 diff: 'infrastructure_pxe' != 'infrastructure_repositories'
E         Left contains more items, first extra item: 'timelines'
```

After
====
```
>       assert set(menu.visible_pages()) == set(group_data)
E       assert set(['about',...ol_log', ...]) == set(['about', ...ol_log', ...])
E         Extra items in the left set:
E         'infrastructure_pxe'
```
Better and correct.