[Diff tests] Replace most visited by a fixed list of pages

[kptdobe]

The most visited request is smart but makes it unpredictable and not necessarily accurate: when we have new customers, we may want to test their sites, even if they are not in the top 20 pages.

I would replace the most visited request by a list managed in a Google Spreadsheet (via helix-home). Like this, we have full control of the sites / pages we want to be tested.

cc @davidnuescheler @MarquiseRosier