Test `#[stable]` versions in CI

[camelid]

Maybe this is an argument for testing since = in CI, I've seen people get it wrong by accident a lot.

Originally posted by @jyn514 in rust-lang/rustc-dev-guide#1036 (comment)

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tidy is probably the right place to do it.

[jyn514]

Note that this should only test #[stable] attributes that are newly added, it makes total sense for old attributes to say an old version.

[jyn514]

And unstable doesn't have an attached version, so it doesn't need to be tested.

[camelid]

And unstable doesn't have an attached version, so it doesn't need to be tested.

I think I confused #[unstable] with language-level feature flags, which do have a version :)