Add support for new VSTestHost when executing 'Test With > In-Proc'

[jcansdale]

NOTE: VisualStudio TestHost project can be used for writing Visual Studio integration tests. As a quick way to prototype a test, the 'Test With > In-Proc' command will populate initialize the VSTestContext class, giving easy to the current ServiceProvider and DTE objects. Click on this issue for an example.

Find out about VSTestHost here:
https://github.com/Microsoft/VisualStudio-TestHost

1. Install VSTestHost MSI.
2. Reference the Microsoft.VisualStudioTools.VSTestHost.14.0 assembly.
3. using Microsoft.VisualStudioTools.VSTestHost
4. Test With > In-Proc on test method. E.g.

void test()
{
    Console.WriteLine(VSTestContext.DTE.Solution.FileName);
}