Check CATKIN_ENABLE_TESTING flag before adding tests

[hidmic]

I came across this small detail almost by pure chance. The catkin build failed in a strange way, complaining about undefined macros that eventually led me to the fact that CMakeLists.txt was not checking whether it should add the test or not.

[bulwahn]

I came across the same issue in the OpenEmbedded ROS Layer, meta-ros, and created a slightly different other patch: https://github.com/bulwahn/urdf_tutorial/commit/e125886b0c497f01541868dc184130e33089f116

My commit also moves the roslaunch dependency into the CATKIN_ENABLE_TESTING part.

[DLu]

Closed in favor of #29