Test Platform and Framework Detection

Determine which test platform (VSTest or Microsoft.Testing.Platform) and which test framework (MSTest, xUnit, NUnit, TUnit) a project uses.

Response contract

When the requested output includes Platform:, report the platform that actually executes tests: VSTest or MTP. Do not put the dotnet test command mode on that line. If command mode matters, report it separately:

dotnet test mode: VSTest
Platform: MTP
Framework: MSTest

Thus an SDK 9 bridged project is Platform: MTP, even though its dotnet test mode is VSTest.