Xunit: Add Support for IAsyncDisposable to Dispose of Test Resources

[RehanSaeed]

netcoreapp3.0 or netstandard3.0 they added IAsyncDisposable. It would be nice if xUnit supported asynchronous disposal of test classes and called DisposeAsync, just like it supports calling Dispose today for IDisposable.

```c#
public class FooTest : IAsyncDisposable
{
[Fact]
public void Foo_When_Then()
{
// ...
}

public ValueTask DisposeAsync()
{
    // ...
}

}
```

[bradwilson]

This is already supported today through IAsyncLifetime, for test classes and fixtures.

[RehanSaeed]

I think it's also useful to use the built-in types which will eventually become the defacto standard for disposing of objects asynchronously. It's also more discoverable.

[bradwilson]

I understand. I'm offering you a mechanism that already works today so you don't have to wait.