Write Behat test for admin_menu

We install navbar by default, but include admin_menu and officially support it in Panopoly. However, since it's not enabled by default, it's easy to miss bugs in upgrades.

Apparently, the admin_menu we currently include in Panopoly doesn't work for the issues described in this issue:

#2420281: Administration menu: errors when enabling it

And we didn't catch it. :-/

So, it'd be nice to have a simple smoke test that:

1. Enables admin_menu
2. Disables navbar
3. Clicks a menu item

We would have caught the issue at step #1 if we had such a test!