Remove non_ascii_idents feature gate from test suite

[dtolnay]

This feature was just stabilized in rust-lang/rust#83799.

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)]`