Make sure you source the virutal environment in all cases to run tests in this way (if you are using a colcon environment, the same commands will eventually hold once ros2's rosdep becomes functional).
Always run tests from the root directory, since that is where
it will pick up the nosetests configuration in setup.cfg.
# All Tests via SetupTools (indirectly)
$ python setup.py nosetests
# All Tests via Nosetest (directly)
$ nosetests -s ./tests
# A single test module
$ nosetests -s tests/test_oneshot.py
# A single test
$ nosetests -s tests/test_blackboard.py:test_activity_stream
If not in the virtualenv and in a dual python environment, e.g. Ubuntu bionic,
replace those commands with their python3 / nosetests3 equivalents.