It doesn't count as a suicide for the dummy account. It hasn't died, just disconnected. So even though you win its not because you have more kills (or less deaths) its just because the other account disconnected. Sorry mate, but short of keeping tabs on a bit of paper you'll be guessing when your 500 comes up.
Maybe do it for half an hour, count how many you've done in that time and then based on the time you know you've put in and those number of matches per half hour you should be able to estimate how many you've done.
(as at Oct 16 2012)