UriFormatException : Invalid URI: Invalid port specified

That’s because you’re executing this code while the pack:// scheme is not yet registered. This scheme is registered when you create the Application object. You can add this code in the setup of your test fixture:

[SetUp]
public void Setup()
{
    if (!UriParser.IsKnownScheme("pack"))
        new System.Windows.Application();
}

EDIT: actually it seems the pack:// scheme is registered in the type initializer of the PackUriHelper class (which happens to be used by the Application class). So actually you don’t need to create an instance of Application, you only need to access a static member of PackUriHelper to ensure the type initializer has run:

[SetUp]
public void Setup()
{
    string s = System.IO.Packaging.PackUriHelper.UriSchemePack;
}