CurrentDirectory not set for dotnet-test based test runners

[jcansdale]

NOTE: The current directory wasn't being set when executing using a 'dotnet test' based test runner. It is now being set to the output directory (consistent with other TestDriven.Net runners).

For example:

        static void CurrentDirectory()
        {
            Console.WriteLine(System.IO.Directory.GetCurrentDirectory());
        }

Was incorrectly showing:

------ Test started: Assembly: BenchmarkDotNet.IntegrationTests.TestDriven.dll ------

C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE

1 passed, 0 failed, 0 skipped, took 1.43 seconds (net451).