Async-streams: Add test for ref struct enumerator

[jcouv]

Should be disallowed in await foreach, as ref struct variables aren't allowed in async methods.

class C
{
    public Enumerator GetAsyncEnumerator()
    {
        return new Enumerator();
    }

    public ref struct Enumerator
    {
        public async Task<bool> MoveNextAsync() => true;
        public int Current => throw null;
        public Task DisposeAsync() { }
    }
}