[rustdoc] Ensure that temporary doctest folder is correctly removed even if doctests failed
Fixes#139899.
The bug was due to the fact that if any doctest fails for any reason, we call `exit` (or it's called inside `libtest` if not edition 2024), meaning that `TempDir`'s destructor isn't called, and therefore the temporary folder isn't cleaned up.
Took me a while to figure out how to reproduce but finally I was able to reproduce the bug with:
`````rust
#![doc(test(attr(deny(warnings))))]
//! ```
//! let a = 12;
//! ```
`````
And then I ensured that panicking doctests were cleaned up as well:
`````rust
//! ```
//! panic!();
//! ```
`````
And finally I checked if it was fixed for merged doctests too (`--edition 2024`).
To make this work, I needed to add a new public function in `libtest` too which would call a function once all tests have been run.
So only issue is: I have absolutely no idea how we can add a regression test for this fix. If anyone has an idea...
r? `@notriddle`
83e3d0e17b