Rollup merge of #140706 - GuillaumeGomez:fix-missing-temp-dir-cleanup, r=notriddle

[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`

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