Enable browser tests for the GettingStarted extension

See also commits in:

mediawiki/selenium
 https://gerrit.wikimedia.org/r/#/c/144978/

zuul-config
 https://gerrit.wikimedia.org/r/#/c/152795/

Change-Id: I2b90c3628b0f7277adb29d567996a108f35bae37
Depends-On: I79d9468e56bc7941c7c5bb27a77d1622bc7d3836