Attribute Macro run_singlethreaded_test

#[run_singlethreaded_test]

Used to run tests that require TestHarness types as inputs on a singlethreaded executor. This attribute should be used instead of #[test], not in addition to it.

e.g.

```
impl TestHarness for SomeHarness {..}

#[test_harness::run_singlethreaded_test]
async fn test_foo(harness: SomeHarness) {
    // use harness
}
```