I have a lump on the top of my right testicle. It's been there for ~13 years with no pain or discomfort. Could this be cancer? What should I do?

Probably not BUT! Why wait so long? Get it examined by your urologist.! by description, it is most likely not a cancerous process. This will depend on your age, and other factors. You may be describing a cyst on the cord, or in the epididymus or a benign structure. It can be simply anatomic confusion. My best advice is to get it checked. Why take the chance. You may need an ultrasound test to help confirm.
Cyst. It's most likely an epididymal cyst. Since its been there for 13+ years, the chances of it being cancer are very small. Check with your doctor for an ultrasound if you're worried.
Why now. If it has been there for so long it is likely benign. Why are you concerned now - did it grow or become painful? If you want a definite answer, consult a urologist, who may choose to do an ultrasound and biopsy.
Unlikely cancer. Probably a spermatocele/cyst of head of epididymis which is quite benign. But, should be checked out even if there is only a remote chance of cancer. Good luck.