[rustdoc] Ensure that temporary doctest folder is correctly removed even if doctests failed

[GuillaumeGomez]

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:

#![doc(test(attr(deny(warnings))))]

//! ```
//! let a = 12;
//! ```

And then I ensured that panicking doctests were cleaned up as well:

//! ```
//! 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

[GuillaumeGomez]

Applied suggestion.