Clean up rustdoc tests by moving them into one place

[GuillaumeGomez]

Fixes#76035

The idea behind this move is to have tests in one place. It became very important for me because we started to have fixtures, making the code "more messy" than it should in my opinion. Like this, all tests are in one place and they're not mixed with the code anymore.

r? @jyn514

I think it'll also interest @ollie27 so pinging them!

[matklad]

Should we just cfg(test) the parent module instead of duplicating the cfgs?

[petrochenkov]

The whole point of Rust unit tests is that they can be placed in modules which they are testing, preventing things that don't need to be public from being public.
No other rustc crate uses the proposed scheme.

[bors]

☔ The latest upstream changes (presumably #73819) made this pull request unmergeable. Please resolve the merge conflicts.

[jyn514]

I agree with petrochenkov here ... this makes a lot of functions public just for testing.

It became very important for me because we started to have fixtures, making the code "more messy" than it should in my opinion.

Can you expand on this? What fixtures does rustdoc use for unit tests?

[jyn514]

Also, you linked to the wrong issue in the PR description, I think you meant #76036.

[jyn514]

I'm going to close this since it's not clear it's a useful change.