Harris should accept the offer on the condition Trump takes the same test. Do it on live TV. PBS to host moderated by Judy Woodruff.
Make it a 60 or 90 minute special, those tests don't take that long. I'd also throw in a few motor skills options, even a six minute walk test. A physician and psychologist selected by a private non partisan medical group. (Someone like the Mayo Clinic, Duke, or whomever). The candidates can be isolated in separate areas. Test one then the other.
I bet Trump would back out.