Browsertests: minor fixes to tags and sitelinks.feature

Added @wikidata.beta.wmflabs.org tag to sitelinks.feature, means that
sitelink tests will now also run regularely on beta.

Introduced a new tag @performance_testing for easy grouping of performance
related tests.

Change-Id: Ifef2d2fc286a0060d03096074a352db967131bb0
Merge "Browsertests: minor fixes to tags and sitelinks.feature"