Fix false positive with missing_docs and #[test]

Since #130025, the compiler don't ignore missing_docs when compiling the tests.
But there is now a false positive warning for every `#[test]`

For example, this code
```rust
//! Crate docs

fn just_a_test() {}
```

Would emit this warning when running `cargo test`

```
warning: missing documentation for a constant
 --> src/lib.rs:5:1
  |
4 | #[test]
  | ------- in this procedural macro expansion
5 | fn just_a_test() {}
  | ^^^^^^^^^^^^^^^^^^^
```

tests/ui/lint/lint-missing-doc-test.rs

@@ -3,3 +3,6 @@
 
 //@ check-pass
 //@ compile-flags: --test -Dmissing_docs
+
+#[test]
+fn test() {}