Fix test dependency issue with moveit_ros_planning

[JeroenDM]

The prerelease tests failed because with the error:

-- Could not find the required component 'moveit_ros_planning'. The following CMake error indicates that you either need to install the package with the same name or change your environment so that it can be found.
CMake Error at /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:83 (find_package):
  Could not find a package configuration file provided by
  "moveit_ros_planning" with any of the following names:

    moveit_ros_planningConfig.cmake
    moveit_ros_planning-config.cmake

  Add the installation prefix of "moveit_ros_planning" to CMAKE_PREFIX_PATH
  or set "moveit_ros_planning_DIR" to a directory containing one of the above
  files.  If "moveit_ros_planning" provides a separate development package or
  SDK, be sure it has been installed.
Call Stack (most recent call first):
  CMakeLists.txt:10 (find_package)

-- Configuring incomplete, errors occurred!

My assumption is that this is because moveit_ros_planning was still under required components in the CMakeLists.txt, but it is only a test dependency in the package.xml. Moving it to the test section of the CMakeLists.txt should solve it, I think. (The tests do depend on it to load the plugin.)

I will try this out in a clean docker container before merging.

Note that I ran the prerelease tests on the branch from #48.

[JeroenDM]

It did work in a docker container started from ros:melodic-ros-core and using rosdep install --from-paths src --ignore-src -r -y to install dependencies. (Where I prebuild and installed opw_kinematics.)

So I'll merge it, rebase pull request #48, and try the prerelease tests again.

[JeroenDM]

O well, it's going to take me a while before I become a git wizard, but I think it worked out here.