Add VS tests for ResolveUnopenedNamespaceTagger

Added 5 VS tests for the feature.

I tried to replace `createSVsResourceManager()` by an Foq instance, but the following call doesn't work:

```fsharp
Mock<IVsResourceManager>()
	.Setup(fun x -> <@ x.LoadResourceBitmap (any(), any(), any(), any()) @>)
	.Returns(VSConstants.S_FALSE)
	.Create()
```

Any idea how to address this one?