`Assert.MultipleAsync -> EnterMultipleScope` autofix produce invalid code

```cs
[Test]
public async ValueTask Test()
{
    await Assert.MultipleAsync(async () =>
    {
        await Task.CompletedTask;
    });
}
```

after fix applied:

```cs
// 1. ValueTask replaced with Task
// 2. duplicate async added
[Test]
public async async Task Test()
{
    using (Assert.EnterMultipleScope())
    {
        await Task.CompletedTask;
    }
}
```