I’ve got to disagree. It does an excellent job of planting in people’s minds the idea that she’s in horrible health and that her people are finding ways of making her appear healthier than she is. It doesn’t matter at all whether or not she’d ever agree to a drug test.
The point is that he’s getting people (even I would bet, many of her supporters) consciously or subconsciously to doubt anything that looks like energy, fight, or drive on her part, especially during the debates (not just the upcoming one but the previous ones too)—and he did it in a very entertaining way. (Humor often helps people accept ideas without fighting back against them in the way they would against serious arguments.)
Trump’s been using a lot of great persuation methods since he announced his run, but ones that not everyone recognizes, so they underestimate the effectiveness of what he does. I think it was a clever move on the very relevant topic of her health (the best sort of “fighting back” since it’s fun rather than vicious, but serves a substantial purpose too)—and I do love to see the press propagandists freaking out over nothing.
It's a masterful bit of subtle slice'n'dice.