BugId:98734 - Server-side test for adsinhead 

When $wgLoadAdsInHead is set, pageviews get into either adsinhead=1 (50%) or adsinhead=2 (50%) group, which get logged to GA (custom variable 39) and if adsinhead=1 JavaScript ad code is pushed up to HTML <head>. This allows us to test the performance hit for pushing ad code up in HTML.