Testhost should use the apphost sdk feature

[ViktorHofer]

testhost.dll should use the apphost sdk feature to crate a native executable testhost.exe which finds the right host itself without using the PATH environment variable. This native executable should be invoked instead of the testhost.dll. This would also solve the problem with looking up the right host for x86 vs x64.

This helps dotnet repositories like corefx to use VSTest and VS Test Explorer as the native executable honors environment variables like DOTNET_ROOT which can be used to point to the right host.

One concern from @vitek-karas was that the apphost feature isn't backwards compatible.

cc @mayankbansal018 @cltshivash @singhsarab