Make sure you have an sourced the appropriate environment (virtual or colcon) before executing.
# run all tests in the current directory
$ pytest-3
# All tests with full stdout (-s / --capture=no)
$ pytest-3 -s
# A single test module
$ pytest-3 -s test_alakazam.py
# A single test
$ pytest-3 -s test_action_clients.py::test_success
# Using tox from the root dir
$ tox -l # list runnable contexts
$ tox # everything
$ tox -e py38 # tests only
$ tox -e flake8 # lint only