using CATKIN_ENABLE_TESTING to optionally configure tests in tf

Since version 0.5.68,  catkin provides to optionally configure
tests. This commit adjusts the CMakeLists.txt to opt-in the tests.
Furthermore, it groups static_transformer_publisher to the other
debug tools and adds some structuring comments in CMakeLists.txt.

This commit is currently needed for cross-compiling tf (without
tests) in the meta-ros project (github.com/bmwcarit/meta-ros)
without requiring gtest. Details are found at
https://github.com/bmwcarit/meta-ros/issues/95.
    
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>