If I get the piss right before the test will it still be warm enough once I have to do the test?
Depends on ambient temperatures outside and inside the facility. If the facility has their A/C jacked up way high, you may have to step up on insulating the sub container and/or use handwarmer packs.

You should conduct practice drills with heating, stashing, and pouring - using warmed water in place of urine. As the "sticky" points out, practice is the key to a successful sub...practice, practice, practice.