Ignore unexpected_cfgs lint for tests

```
error: unexpected `cfg` condition name: `rustfmt`
 --> src/gen/assert_impl.rs:6:13
  |
6 | #![cfg_attr(rustfmt, rustfmt::skip)]
  |             ^^^^^^^
```

Due to rust-lang/rust#124735, we cannot
disable this lint at module-level.

Author: taiki-e

src/lib.rs

@@ -132,6 +132,7 @@ Note: The MSRV when these features are enabled depends on the MSRV of these crat
 )]
 // docs.rs only (cfg is enabled by docs.rs, not build script)
 #![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(test, allow(unexpected_cfgs))] // https://github.com/rust-lang/rust/issues/124735
 
 #[cfg(test)]
 #[path = "gen/assert_impl.rs"]

tools/codegen/src/file.rs

@@ -33,7 +33,8 @@ pub(crate) fn workspace_root() -> PathBuf {
 #[track_caller]
 pub(crate) fn header(function_name: &str) -> String {
     // rust-analyzer does not respect outer attribute (#[rustfmt::skip]) on
-    // a module without a body. So use inner attribute under cfg(rustfmt).
+    // a module without a body and unstable ignore option in .rustfmt.toml.
+    // So use inner attribute under cfg(rustfmt).
     format!(
         "// SPDX-License-Identifier: Apache-2.0 OR MIT
 // This file is @generated by {bin_name}