I found a way to get 0:00:00 on a test.
If you go off a jump and land past where the test ends and keep on driving down the track eventually you will get 0:00:00 and a gold medal.
this is easiest to do on the sand ocean catapillar tests but not the full lap test.
if you do it on a full lap test you have to run the test again.
that's really old. it was known since months after the game was released.
that glitch is banned in the ladder. that's why you don't see 0 second times there.