GA2: Running the Auto-grading Tests for Mac

[kmonahan]

At least one learner and I have encountered that the instructions and screenshots in "Running the Auto-grading Tests" do not match what we see in Visual Studio for Mac.

1. (minor) "Run Unit Tests" is located under the Debug menu, not the Run menu
2. (more important) No tests can be detected and run until the project compiles successfully, which requires commenting out TestTask3.cs until TestTask2.cs is completed. Suggest starting with TestTask3.cs commented out, as the other future tasks are, and updating the instructions accordingly.