That's the key question that need to be answerd: Does Pirelli's contract overrule the rules? If so, is it ok that Merc tested with their current car?You have to look at all regulations. Some overrule others.
And according to Pirelli, Mercedes has no idea what tires they tested. They just had codes on them.
And besides of that I find it quiet interesting that out of all teams Merc was selected.