Actually You might have kicked yourself there. Have a read at wikipedia about urine_samples and detection times.
1. If the test is to diluted they might say, no go and notify the person who ordered the test. (just say u drank alot that day)
2. If they suspect u have used some dilution "thing" they will just add som of the substance they are testing for to a small sample. If the test still goes negative it contains a dilution substance and they will notify the guy who pays for the test.
3. The detectiontimes for thc(cannabinoids) is 2-3 days for infrequent users, and up to 10 days(this is hardly ever seen, and only inn heavy frequent users)
All according to wikipedia/drug_test(with link to the researchpaper that makes it a fact)

Sorry to be the bearer of bad news. Hope it went ok anyhow though.