QA: Refactored performance tests

Fixed up feature indentation and clauses and moved global methods into
the page-object classes.

Added @custom-browser tag to avoid starting multiple browser sessions
and updated mediawiki_selenium dependency for fix to custom browser
instantiation with SauceLabs.

Change-Id: I9741afb8d13ee8729f72210eb7617d9446311a8e
Merge "QA: Refactored performance tests"