Rename 'tests' directory to 'behat'

This is something that's been bothering me for a long time! We used to have the directory structure panopoly_test/tests/behat/ which was a lot to type (especially because there was nothing else in the 'panopoly_test/tests/' directory) so it got simplified to panopoly_test/tests/ but... everything in there is Behat specific.

I think I'd prefer using panopoly_test/behat/ so it's clear what all is in there!