Remove non_ascii_idents feature gate from test suite

error: the feature `non_ascii_idents` has been stable since 1.53.0 and no longer requires an attribute to enable
     --> test_suite/tests/test_gen.rs:6:43
      |
    6 | #![cfg_attr(feature = "unstable", feature(non_ascii_idents))]
      |                                           ^^^^^^^^^^^^^^^^
      |
    note: the lint level is defined here
     --> test_suite/tests/test_gen.rs:5:9
      |
    5 | #![deny(warnings)]
      |         ^^^^^^^^
      = note: `#[deny(stable_features)]` implied by `#[deny(warnings)]`

test_suite/tests/test_gen.rs

@@ -3,7 +3,6 @@
 // types involved.
 
 #![deny(warnings)]
-#![cfg_attr(feature = "unstable", feature(non_ascii_idents))]
 #![allow(
     unknown_lints,
     mixed_script_confusables,
@@ -267,7 +266,6 @@ fn test_gen() {
     }
     assert::<EmptyEnumVariant>();
 
-    #[cfg(feature = "unstable")]
     #[derive(Serialize, Deserialize)]
     struct NonAsciiIdents {
         σ: f64,