Which specific tests? Would say that the depression/anxiety/panic ones are probably pretty accurate. As far as autism, personality disorders, and so forth those diagnoses can be much more complex so I wouldn't trust them as much.

I think the quizzes are best used as rough guides - like if a quiz suggests you may have a particular issue then check out more stuff online about that diagnosis and see if you feel it fits after further research (wikipedia is usually a good place to start). If you think it might fit after more research you might make a post on the appropriate sub-forum to get the opinion of the people there who are diagnosed and probably very knowledgeable.

Then maybe bring it up with your doctor while referencing the DSM criteria and your relatable experiences instead of just a quiz that he/she will have no knowledge of and definitely not trust in any way. If I were to take an ASD quiz, a schizoid personality quiz, an avoidant personality quiz, and a BPII quiz I would probably test either "potentially have this" or "definitely have this" for all of them but it's not really likely or possible for that to be true... they are very far from accurate and conclusive because it's impossible to differential diagnose with such simple quizzes. But do use them as a reason to investigate a diagnosis more and then make a call based on the evidence you find whether it's worth bringing up with you psych or researching further.