dotnet test with source assembly doesn't set DOTNET_ROOT

[ViktorHofer]

3.1 introduced support for using dotnet test to invoke a test project or an assembly. This means that dotnet vstest which was used to invoke a test assembly is de facto deprecated as it's replaced by dotnet test. Testing a test assembly with dotnet test on Windows results in the testhost (which uses the apphost feature) not being able to find the runtime because DOTNET_ROOT isn't set.

cc @nohwnd

[ViktorHofer]

Discussed this in tactics today. We need to assemble more data to understand when this regressed and why there aren't any reports of it (my assumption: it might not be common to use dotnet test on test assemblies yet - probably because docs weren't updated back when that support was added).

Let's come up with the change for master and then discuss backporting.