Pytest warnings about unknown marks

[emersonknapp]

Pytest for the linters has started emitting PytestUnknownMarkWarnings

I've started seeing these warnings in my development environment as well. Based on the error output, it seems that we can register the linter marks and stop these warnings.

Sample output from https://ci.ros2.org/job/ci_osx/6068/console

17:29:00 =============================== warnings summary ===============================
17:29:00 /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337
17:29:00   /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337: PytestUnknownMarkWarning: Unknown pytest.mark.copyright - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
17:29:00     PytestUnknownMarkWarning,
17:29:00 
17:29:00 /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337
17:29:00   /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337: PytestUnknownMarkWarning: Unknown pytest.mark.linter - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
17:29:00     PytestUnknownMarkWarning,
17:29:00 
17:29:00 /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337
17:29:00   /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337: PytestUnknownMarkWarning: Unknown pytest.mark.flake8 - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
17:29:00     PytestUnknownMarkWarning,
17:29:00 
17:29:00 /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337
17:29:00   /Users/osrf/jenkins-agent/workspace/ci_osx/venv/lib/python3.7/site-packages/_pytest/mark/structures.py:337: PytestUnknownMarkWarning: Unknown pytest.mark.pep257 - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
17:29:00     PytestUnknownMarkWarning,

[dirk-thomas]

Please see #164.