Rustdoc gui tests
This is a reopening of #70533.
For this first version, there will be no screenshot comparison. Also, a big change compared to the previous version: the tests are now hosted in the rust repository directly. Since there is no image, it's pretty lightweight to say the least.
So now, only remains the nodejs script to run the tests and the tests themselves. Just one thing is missing: where should I put the documentation for these tests? I'm not sure where would be the best place for that. The doc will contain important information like the documentation of the framework used and how to install it (`npm install browser-ui-test`, but still needs to be put somewhere so no one is lost).
We'd also need to install the package when running the CI too. For now, it runs as long as we have nodejs installed, but I think we don't it to run in all nodejs targets?
cc `@jyn514`
r? `@Mark-Simulacrum`
352238d152