I think it depends on what comes out in the evidence.
if it is clear that Pirelli approached Merc about doing a test and Merc went to the FIA (albeit possibly just a quick chat about it and didn’t get all the boxes ticked) and its clear they didn’t run any new parts or gather any car data other than was required for the testing then Merc will be OK and Pirelli will face whatever sanction you can give to the sole tyre supplier.
If however it transpires that Merc went to Pirelli and said along the lines of ‘if you invite us for a test nudge nudge, me and you can do 1000km of testing nudge nudge’ and then run a whole load of prototype parts and have all the airflow measuring kit on etc then it could get quite nasty for Merc.
That said, credit to Brawn – he said at Canada that the decision to run the test was his and if there are any repercussions then on his head be it but he genuinely doesn’t seem to think Merc have done anything wrong.
Cheers
Danny B