Xcode: Comply with the instructions for subclassing XCTestObserver

/*! Sent immediately after running tests to inform the observer that it's time 
    to stop observing test progress. Subclasses can override this method, but 
    they must invoke super's implementation. */
- (void) stopObserving;
Xcode: Open external Xcoverage tool only if it's installed
Assignment 3: Prepare test harness for playing cards
Assignment 3: Implement test cases for playing cards