Tracking issue for release notes of #131729: Make the test cfg a userspace check-cfg

[rustbot]

This issue tracks the release notes text for #131729.

Steps

• Proposed text is drafted by PR author (or team) making the noteworthy change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text

The responsible team for the underlying change should edit this section to replace the automatically generated link with a succinct description of what changed, drawing upon text proposed by the author (either in discussion or through direct editing).

# Compatibility notes
- [`rustc` no longer treats the `test` cfg as a well known check-cfg](https://github.com/rust-lang/rust/pull/131729), instead it is up to the build systems and users of `--check-cfg`[^check-cfg] to set it as a well known cfg using `--check-cfg=cfg(test)`.
  
  This is done to enable build systems like Cargo to set it conditionally, as not all source files are suitable for unit tests.
  [Cargo (for now) unconditionally sets the `test` cfg as a well known cfg](https://github.com/rust-lang/cargo/pull/14963).

[^check-cfg]: https://doc.rust-lang.org/nightly/rustc/check-cfg.html

Tip

Use the previous releases categories to help choose which one(s) to use.
The category will be de-duplicated with all the other ones by the release team.

More than one section can be included if needed.

Release blog section

If the change is notable enough for inclusion in the blog post, the responsible team should add content to this section.
Otherwise leave it empty.

cc @Urgau, @petrochenkov -- origin issue/PR authors and assignees for starting to draft text