Moved tests from browsertests repository

See also the commit[1] that deletes the tests from browsertests
repository and the commit[2] in mediawiki/selenium repository that
documents changes in Jenkins jobs.

The tests run just fine on my machine. Added CREDITS file 
following advice from Luis Villa[3].

1: https://gerrit.wikimedia.org/r/#/c/113115/
2: https://gerrit.wikimedia.org/r/#/c/113138/
3: https://bugzilla.wikimedia.org/show_bug.cgi?id=49313#c2

Bug: 61311
Change-Id: Id293a6bb06430bd49334c2964a4c2a28e175182a
Merge "Moved tests from browsertests repository"