[Browser test] Headless browser test(s)

Ready for code review!

New headless browser test for use on a fresh Mediawiki install that
includes VisualEditor.  Using the gem mediawiki_api

Updated to use the create-user API method from the
mediawiki_api gem.

Bug: 53691
Change-Id: I07d4ae5fac346b0c2ee8c33a2dc2456299047ace