[BrowserTest] Language Screenshot feature needs site specific tag

Fixing failed Jenkins job

bundle exec cucumber … --tags @en.wikipedia.beta.wmflabs.org --tags
@firefox --tags @language_screenshot
…
0 scenarios
0 steps
0m0.000s
…

https://integration.wikimedia.org/ci/job/browsertests-VisualEditor-langu
age-screenshot-linux-firefox-sauce/LANGUAGE_SCREENSHOT_CODE=en,label=con
tintLabsSlave/1/console

Bug: 62737
Change-Id: I2da00dfae29fa6d51c45864caea42d39ec60983d