using CATKIN_ENABLE_TESTING to optionally configure tests

Since ff60599b8077b86a6d75d510da72e69899d16008 in the catkin
repository, catkin provides means to optionally configure tests.
This commit adjusts the CMakeLists.txt to opt-in the tests.

This commit is currently needed for cross-compiling class-loader
(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/86.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>