Support attributes in subtests

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 `[]` 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:
```rust
#[neotest::test]
fn test_vec_index() {
let vec: Vec = 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.

Start by reading the `subtest! { ... }` macro and the surrounding test framework entry points. Compare the proposed attribute-oriented DSL and function-based syntax, including how subtest scope would work. Done requires a decided approach that supports attributes such as `#[should_panic]` and `#[ignore]` for subtests.

Written by the indexing model from the issue text.