Ignore non_local_definitions false positive in test

rust-lang/rust#121621

    warning: non-local `impl` definition, they should be avoided as they go against expectation
        --> tests/test.rs:2338:5
         |
    2338 | /     impl<'de> Deserialize<'de> for &'de RawMapKey {
    2339 | |         fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    2340 | |         where
    2341 | |             D: serde::Deserializer<'de>,
    ...    |
    2345 | |         }
    2346 | |     }
         | |_____^
         |
         = help: move this `impl` block outside the of the current function `test_raw_value_in_map_key`
         = note: an `impl` definition is non-local if it is nested inside an item and neither the type nor the trait are at the same nesting level as the `impl` block
         = note: one exception to the rule are anon-const (`const _: () = { ... }`) at top-level module and anon-const at the same nesting as the trait or type
         = note: this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <rust-lang/rust#120363>
         = note: `#[warn(non_local_definitions)]` on by default

tests/test.rs

@@ -2335,6 +2335,8 @@ fn test_raw_value_in_map_key() {
     #[repr(transparent)]
     struct RawMapKey(RawValue);
 
+    #[allow(unknown_lints)]
+    #[allow(non_local_definitions)] // false positive: https://github.com/rust-lang/rust/issues/121621
     impl<'de> Deserialize<'de> for &'de RawMapKey {
         fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
         where