I have no problem with this. Drug testing is an integral part of the hiring process for many jobs today and the Presidency is an extremely important job. Certainly, if a candidate was using drugs to enhance their performance or stamina in a debate I think we should know that so we can factor that into our decision making process. More importantly and just off the top of my head, if a candidate were to have a serious illness let's say Parkinson's, I think we should know this before hitting the voting booth.
Trump has been accused and right here too of using cocaine before the first two debates. Let's find out if he's using now. Seems like a smart thing to do.
I see nothing unreasonable about doing drug tests in this case.
Have you considered asking yourself why you would bother asking the president (or candidate) or premier of any country to undergo drug testing? Considering that it's not their job to do anything remotely Olympic, and that in the world's most visible job roles it would be freaking obvious if they took anything that affected their mental faculties. Performance in a debate is not about how many words you can say per minute, or how loudly you can say them. I'm not aware of any drugs that would increase a candidate's awareness of how to engage an interactive audience with care and engage in all the subtle means at one's disposal to ingratiate yourself with them, or hone one's judgement of when to tone down one's approach and change tactics if one's current strategy isn't working.
Furthermore, how do you propose "drug testing" is going to catch Parkinson's disease exactly? Or if the condition was sufficiently advanced to warrant diagnosis and medication, why on earth a presidential candidate would want to put themselves in a position of increased visibility so that the physical indicators are more likely to be noticed? Just how invasive would you want to make the health screening? Does a person's increased risk of developing a particular disorder rule them out immediately? Have you considered that probably every single presidential candidate is probably at increased risk of a load of stress-encouraged disorders, and that many (most?) disorders are exacerbated by stress? Or that presidential candidates are likely to be an age that's advanced enough to have allowed them to have sufficient experience of the politics they're likely to encounter as president, yet also that age is advanced enough to put them at increased risk of just about everything?
If Trump wants to use cocaine during presidential debates, let him! Apart from dropping some speed or LSD, or chasing half a bottle of whisky with paracetamol, I can't think of many better ways to torpedo his own campaign.