Expose public APIs for end-to-end SDK tests

The SDK repo makes use of an internal `SdkTest` API to support writing end-to-end tests that validate the behavior of `dotnet` in the context of a project. The `Microsoft.NET.TestFramework` that encapsulates this APIs is not shipped anywhere publically, as far as I can tell.

Becuase there's no common abstraction for writing these kinds of end-to-end tests against a project, a crop of solutions have emerged. The ones that I am aware:

- Individual APIs for validating templates shipped out of the ASP.NET shared framework
- MSBuild-based test scaffolding for trimming and native AoT tests in dotnet/aspnetcore and dotnet/runtime

A public testing API would reduce the maintaince burden required to maintain multiple testing techniques in the same repo and reduce the barriers for writing end-to-end tests for projects in codebases.

cc: @baronfel who encouraged me to file this issue after I tried to take some MSBuild-based testing infrastructure past its limits 😅