Fix -Wunused-but-set-variable in transfer function tests

Since the tests only exercise the method to make sure there is no
crash, adding "Q_UNUSED" is good enough.

Fixes #209

Libs/Visualization/VTK/Core/Testing/Cpp/ctkVTKColorTransferFunctionTest1.cpp

@@ -205,6 +205,7 @@ int ctkVTKColorTransferFunctionTest1( int argc, char * argv [])
   defaultControlPoint = defaultTF.controlPoint(0);
   defaultControlPoint = defaultTF.controlPoint(firstIndex);
   defaultControlPoint = defaultTF.controlPoint(secondIndex);
+  Q_UNUSED(defaultControlPoint)
 
   colorTransferFunction->Delete();
   return EXIT_SUCCESS;

Libs/Visualization/VTK/Core/Testing/Cpp/ctkVTKTransferFunctionRepresentationTest1.cpp

@@ -50,6 +50,7 @@ int ctkVTKTransferFunctionRepresentationTest1( int argc, char * argv [])
   defaultControlPoint = defaultCTF.controlPoint(0);
   defaultControlPoint = defaultCTF.controlPoint(firstIndex);
   defaultControlPoint = defaultCTF.controlPoint(secondIndex);
+  Q_UNUSED(defaultControlPoint)
 
   representation.computeCurve();
   representation.computeGradient();