Fix catkin failed build when testing is not enabled

[MCFurry]

On a clean catkin_build I get thrown this error:

Errors     << steering_functions:check /home/mcfurry/ros/melodic/system/logs/steering_functions/build.check.000.log                                                                                               
CMake Error at /opt/ros/melodic/share/catkin/cmake/test/tests.cmake:18 (message):
  () 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:207 (catkin_add_gtest)

make: *** [cmake_check_build_system] Error 1

This fixes this.

[hbanzhaf]

I never experienced that error before. How about you, @Timple?

[Timple]

The error seemed self-explanatory. But interestingly it doesn't reproduce in a default setting. Tested using Dockerfile below:

FROM ros:noetic
RUN apt-get update -qq && apt-get install -qqy python3-catkin-tools python3-osrf-pycommon git
RUN git clone https://github.com/hbanzhaf/steering_functions src/steering_functions

# Install dependencies used by packages
RUN rosdep update && rosdep install -y -i --from-paths src

# Compile ROS package
RUN bash -c 'source /opt/ros/*/setup.bash && catkin build'

But this layout is a-typical for a ROS package (see catkin_lint output). So probably doesn't hurt to conform to the standard to prevent issues in edge-cases.

[MCFurry]

The catkin build command by default enables testing, but in several usecases testing is not enabled. Hence it reproduces with:

FROM ros:melodic
RUN apt-get update -qq && apt-get install -qqy python-catkin-tools git
RUN git clone https://github.com/hbanzhaf/steering_functions src/steering_functions

# Install dependencies used by packages
RUN rosdep update && rosdep install -y -i --from-paths src

# Compile ROS package
RUN bash -c 'source /opt/ros/melodic/setup.bash && catkin build -DCATKIN_ENABLE_TESTING=off'

[Timple]

If the default is ON, it's probably safe to merge this then. No regressions for current users (default ON). And for users who have testing OFF, it failed anyway to compile.