PresentationCore.dll doesn't get picked up by test apps

When using the pattern described in the [developer guide](https://github.com/dotnet/wpf/blob/master/Documentation/developer-guide.md#testing-api-changes) most managed assemblies like `WindowsBase` and `PresentationFramework` will be copied and used, except for `PresentationCore`.

There's probably something small here that just requires investigating the MSBuild bin logs and trying to figure out where these references get picked up and why this is special.