Export MEDIAWIKI_API_URL when running browser tests

There are a lot of failed ContentTranslation tests with error message:

“Environment variable MEDIAWIKI_API_URL must be set in order to create
a target page for this test (SystemExit)”

https://integration.wikimedia.org/ci/view/BrowserTests/job/browsertests-ContentTranslation-language-stage.wmflabs.org-linux-firefox/17/consoleFull

Change-Id: I99b86e1a8c3cd9d738c2b2a244940e7b8954cf8e