Support attributes in subtests

[bitwizeshift]

The current mechanism for subtests simply acts as a series of statements defined in a subtest! { ... } macro. Although this functionally works, it suffers the significant drawback that it is unable to have custom attributes specified -- such as #[should_panic]or#[ignore]`, which are needed for testing.

It's unclear what the best direction forward for this is. A couple thoughts come to mind:

• Extend the DSL to support [<attribute>] syntax, e.g. subtest! { |name| [should_panic][ignore] ... }, this is kind of gross -- but would work.

• Change the syntax entirely; perhaps using function definitions instead:

  #[neotest::test]
  fn test_vec_index() {
    let vec: Vec<u8> = Vec::default();
  
    #[neotest::subtest]
    #[should_panic]
    fn out_of_bounds_panics() {
        vec[20];
    }
  }

  The downside to this approach is that it's not clear that vec is within fn out_of_bounds_panics() scope, since this typically is not a true closure. The upside is that this has a clear mechanism for identifying attributes, the test ident, etc.

The exact approach is yet to be determined.