Add test case for picking up Async methods declared in base classes or interfaces

[samcook]

Currently not picking up async methods declared in base classes or interfaces

private IThing iThing;
private Thing thing;
private Widget widget;

[RewriteAsync]
public void Foo()
{
    iThing.DoSomethingNested();
    iThing.DoSomethingNotNested();

    thing.DoSomethingNested();
    thing.DoSomethingNotNested();

    widget.DoSomethingNested();
    widget.DoSomethingNotNested();
}

becomes

public async Task FooAsync(CancellationToken cancellationToken)
{
    iThing.DoSomethingNested(); // should be async
    await iThing.DoSomethingNotNestedAsync().ConfigureAwait(false);
    await thing.DoSomethingNestedAsync().ConfigureAwait(false);
    await thing.DoSomethingNotNestedAsync().ConfigureAwait(false);
    widget.DoSomethingNested(); // should be async
    await widget.DoSomethingNotNestedAsync().ConfigureAwait(false);
}