Dedicated automated testing page

[andy-hook]

It would be helpful to provide some type of documentation regarding automated testing and primitives, especially with suites running testing-library and user-event. There are quite a few gotchas, especially common are pointer event firing problems and mocking layout specific browser apis as jsdom struggles with these.

Examples of common questions:

radix-ui/primitives#1382
radix-ui/primitives#1220
radix-ui/primitives#1207
radix-ui/primitives#1036
radix-ui/primitives#856
radix-ui/primitives#765
radix-ui/primitives#420

Document null during testing
radix-ui/primitives#1707