Added CTest command --print-labels

This command allows a user to quickly see the list of all available
test labels.  The labels are also printed in verbose show only mode,
alongside their corresponding tests.

Source/CTest/cmCTestMultiProcessHandler.h

@@ -39,6 +39,7 @@ public:
   void SetParallelLevel(size_t);
   virtual void RunTests();
   void PrintTestList();
+  void PrintLabels();
 
   void SetPassFailVectors(std::vector<cmStdString>* passed,
                           std::vector<cmStdString>* failed)