build fails if CATKIN_ENABLE_TESTING is false

[ftsell]

When building the package and supplying -DCATKIN_ENABLE_TESTING=0 cmake fails with error

() is not available when tests are not enabled.  The CMake code should only                                                                                                                                                                                 
  use it inside a conditional block which checks that testing is enabled:                                                                                                                                                                                     
                                                                                                                                                                                                                                                              
  if(CATKIN_ENABLE_TESTING)                                                                                                                                                                                                                                   
                                                                                                                                                                                                                                                              
    (...)                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                              
  endif()                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                              
Call Stack (most recent call first):                                                                                                                                                                                                                          
  CMakeLists.txt:10 (catkin_add_nosetests)

which basically means that the cmake call to catkin_add_nosetests(..) should be inside an if block.