make rostest build dependency in CMakeLists.txt optional

[bulwahn]

We want to make the rostest build dependency in the CMakeLists.txt of all ROS packages optional.

This shall been done by the refactoring the CMakeLists.txt from:
find_package(catkin REQUIRED COMPONENTS [...] roscpp rostest) [...] if(CATKIN_ENABLE_TESTING) [...] endif()

to:

find_package(catkin REQUIRED COMPONENTS [...] roscpp) [! drop rostest !] [...] if(CATKIN_ENABLE_TESTING) find_package(rostest) [! add this line in the CATKIN_ENABLE_TESTING conditional block !] [...] endif()

For commits addressing this task, their commit message shall refer to this issue by
make rostest in CMakeLists optional (ros/catkin#588).

This has been discussed on the ros-sig-buildsystem [1], and prepares for the roll-out of the REP 140 [2].

[1] https://groups.google.com/forum/#!topic/ros-sig-buildsystem/Ub7ktrLPSe0
[2] https://github.com/jack-oquin/rep/blob/master/rep-0140.rst

[dirk-thomas]

We should not list that under catkin but under ros/rosdistro. I will move your ticket there.

[bulwahn]

Okay. Go ahead and please update the commit message to refer to the new issue.