Engine doesn't need to be running, just ignition on, but isn't this anyway.
Coilpack most DEFINITELY can fail on just one plug output, I know had me scratching my head as well, but I've seen 3 like this inc one of my own.
Is the fuel filter change up to date, have you checked the CO(exhaust gas) to see if running lean, look for any air leaks, also it would be a good idea to clean the MAF, with carb cleaner, do NOT under any circumstances poke at the sensor wires, after cleaning reset the ecu by pulling the fuse marked KAM for 1 minute.
There is also another(cheap) part that can cause this commonly, not sure this is on your year though, looking at the front of the engine behind the manifold there is a pipe that leads from the PCV valve to the rear of the head, around the rear corner of the head it has a "T" piece these are famous for disintergrating, if you just temporarily pull off the pipe leading to the inlet from this "T" piece and block it you will see if it has been the problem, as I say not 100% sure this is on yours.
tabetha