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