Cats can and do get colds, they are also susceptible to various viruses that mimic colds; calicivirus, cat flu, rhinotracheitis, panleukopena, herpes, and allergies. Your vet sounds like he errs on the side of caution. With a blood panel, he can rule out certain diseases and with a urine test he can find out if the urine is loaded with bacteria. If your cat isn't up on its vaccinations and you can afford it, I would certainly listen to your vet, for it sounds like he might suspect something besides just an URI. If it is just a common cold, antibiotics should do the trick.
I would talk to your vet and ask him why he wants those extra tests? What does he suspect might be going on? Is your cat eating, drinking, peeing and pooping in the litter pan. active and alert? I would just make a list of questions, go in and see your vet and talk to him. Find out the reasons behind the lab tests, sometimes they see things going on with our cats and forget to clue us in.
You can also help your kitty to breath by buying a baby vaporizer and putting a small amount of vicks in it and just running it so she can catch a breath. Saline solution in the nose will help as well. Take a warm damp cloth, press it to the nostrils for just a few seconds to loosen any crusty discharge, then gently with your finger wipe away the crust. If it is liquid discharge, just wipe that with a clean cloth. Apply the drops (the cat will sneeze) then reapply the drops again.
You can also take kitty into the bathroom, run a hot, hot shower, let it get all steamed up and sit with her while she breathes in the steam. URI are extremely contagious, so if you have other cats, keep her separated until she is healthy again-
Best of luck