Rollup merge of #140673 - GuillaumeGomez:clean-rustdoc-tests, r=notriddle

Clean rustdoc tests folder

We were starting to have way too many tests in the `tests/rustdoc/` folder so I moved some of them in sub-folders. We now have less than 300 tests at the "top level" so I guess it's good enough for now.

So this PR just moves tests in sub-folders and that's pretty much it. 😃

r? ``@notriddle``

Author: Zalathar