Add regression test to dotnet sdk for cpp authoring scenario

[j0shuams]

When getting together project support, we had a timing issue of two PRs needing two land before a regression test can be introduced to the sdk. The changes have been manually tested, and a draft PR for the regression test has been made here. The regression test is a C++ app with two project references to two CsWinRT components. We should just need a test task that builds the solution and makes sure it runs fine. There are some examples of this in the other tests already.