Nosetest is the framework for running Python unit tests. No ROS environment is available. See: Configuring rostest if your tests need a running roscore.
Declare each nose test like this:
if (CATKIN_ENABLE_TESTING)
catkin_add_nosetests(test_your_node tests/test_your_node.py)
endif()
This example assumes your tests are defined in the tests/ subdirectory in your source tree.