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() {}
  | ^^^^^^^^^^^^^^^^^^^
```

compiler/rustc_builtin_macros/src/test.rs

@@ -277,6 +277,8 @@ pub(crate) fn expand_test_or_bench(
                 cx.attr_nested_word(sym::cfg, sym::test, attr_sp),
                 // #[rustc_test_marker = "test_case_sort_key"]
                 cx.attr_name_value_str(sym::rustc_test_marker, test_path_symbol, attr_sp),
+                // #[allow(missing_docs)]
+                cx.attr_nested_word(sym::allow, sym::missing_docs, attr_sp),
             ],
             // const $ident: test::TestDescAndFn =
             ast::ItemKind::Const(