Separate test app SDK from product build SDK

[hoyosjs]

Right now, we build all our profilee and debuggee apps with the same SDK as we do product bits. This is problematic since we want a somewhat stable SDK for the product bits as we ship off band, while retaining the ability to test newer runtimes and TFMs.

We already drop a newer SDK into the dotnet-test folder under artifacts for use in tests. That same SDK can/should be used to build the test apps.