QA: Deal with failing cloudbees test

The language overlay loads asynchoronously. Thus we must delay
the back button press until the language overlay has fully
initialised and can be seen to ensure this test always passes.

See: https://wmf.ci.cloudbees.com/job/
MobileFrontend-en.m.wikipedia.beta.wmflabs.org-linux-chrome
/375/testReport/junit/(root)/Language%20selection/
Closing_language_overlay__browser_button_/

Change-Id: I506d61a6385928db49578d982e78ceb08cc33bc5
Merge "QA: Deal with failing cloudbees test"