Re-enable testing of nightly proc macro

[dtolnay]

No description provided.

[dtolnay]

This is currently failing due to rust-lang/rust#50942.

error: lifetimes cannot use keyword names
  --> test_suite/tests/test_gen.rs:46:25
   |
46 |     #[derive(Serialize, Deserialize)]
   |                         ^^^^^^^^^^^

error: lifetimes cannot use keyword names
  --> test_suite/tests/test_gen.rs:54:25
   |
54 |     #[derive(Serialize, Deserialize)]
   |                         ^^^^^^^^^^^

error: lifetimes cannot use keyword names
  --> test_suite/tests/test_gen.rs:62:25
   |
62 |     #[derive(Serialize, Deserialize)]
   |                         ^^^^^^^^^^^

[dtolnay]

Fixed in rust-lang/rust#50946.