[QA] browser tests race condition with login/logout

[Jon Robson]

> I guess whenever we logout, all that username sessions are invalidated
> on the server side.

That is correct and apparently by design. [1]

>
> The fix would be to use different username probably. Or have the per
> user session invalidation to only invalidate the given session id and
> not per username (havent looked at mediawiki core code).
I think this would make sense. Is it possible to do this easily? e.g.
have users such as selenium-mobile etc

[1] https://bugzilla.wikimedia.org/show_bug.cgi?id=49890