Python warnings are not reported by CI

[jacobperron]

Neither colcon test nor Jenkins report warnings coming from Python code. For example, deprecated API was recently introduced in https://github.com/ros2/ros2cli/pull/463 but CI passes without reported the warnings. I would expect an unstable build so that we notice things like deprecated Python APIs and can address them.

Here is an example build testing ros2node:

It is unstable for a different reason. The point is there are 61 warnings visible in the console, but they are not reported by Jenkins: https://ci.ros2.org/job/ci_linux/9731/consoleFull#console-section-223

Specifically for tests, I tried passing a pytest argument to treat warnings as errors (https://github.com/ros2/ci/pull/416), but this results in a failed build, instead of unstable, which I think is not desired.

Related colcon PR to report pytest warnings: https://github.com/colcon/colcon-core/pull/236