So, you knew you could be drug tested for your job and you smoked anyways? What a dumbass. Just let the results come in, they'll find out that you had THC in your system anyways when the test comes back whether you quit or not. It doesn't sound like you'll be able to use the company as a reference anyways, quitting after a drug screen in pretty obvious behavior.
Who knows, you could get lucky and the test could come out negative. Most of those tests are biased towards giving false negatives instead of false positives, becuase a false positive means a lawsuit.