Is it permissible for a Muslim to undergo medical tests (blood sample, urine and stool analysis, and semen analysis) without a medical necessity or a complaint of illness?

There is no harm in undergoing medical tests for the purpose of reassuring one's health and preventing diseases, even if there is no urgent necessity. The existence of a potential need is sufficient. There is no objection to sacrificing a small amount of blood for this purpose, such as a doctor viewing medical samples.