Logistration: Bok-choy tests for the combined login/registration page.

* Enable the combined login/registration page in the bok choy settings.
* Set the country field in the registration page to "required" to match edx.org
* Add tests for the combined login/registration page.

Note: I've made this PR into a feature branch, since #5948 has not yet been merged to master.

Reviewers: @benpatterson @rlucioni 
One note: is it possible to give this file a better name? It looks like it's been around for a while as `register.py`, but that name no longer accurately reflects the page defined within.