QA: The first Selenium test: CentralAuth log in

Moved from qa/browsertests repository[1]. See also [2].

Added basic, single page test for Special:CentralAuth.

Paired with: Chris Steipp
1: https://gerrit.wikimedia.org/r/#/c/102294/
2: https://gerrit.wikimedia.org/r/#/c/125738/

Change-Id: I71caac5f77a30d1c62cc7fc004c34d519d2a3883