Deny #[test] fns declared in nested items

[udoprog]

This denies #[test] fns declared inside of other nested items, like these:

fn function() {
    #[test]
    fn test_fn() {
        assert!(true != true);
    }
}

The above would fail to compile with:

  ┌─ scripts\test.rn:2:5
  │
1 │   fn function() {
  │   ------------- nested in here
2 │ ╭     #[test]
3 │ │     fn test_fn() {
  │ ╰────────────────^ #[test] attributes are not supported on nested items

TODO

We still need #[test] fns to be fully pruned during compilation in case we're not compiling for a test case.