Python test failures in ign-math don't show up in jenkins test results

[scpeters]

Currently the python tests in ign-math are not generating JUNIT result files, so Jenkins is unable to detect failures of these tests. For example, in https://github.com/ignitionrobotics/ign-math/pull/407#issuecomment-1093652832 it was noted that a python test fails on 20.04, and while the GitHub action for Ubuntu Focal shows a failure, the Jenkins job indicates success:

• https://build.osrfoundation.org/job/ignition_math-ci-pr_any-ubuntu_auto-amd64/1318/

Inspection of the build log shows that the python test has indeed failed, but none of the python tests, whether they pass or fail, are registered in the test_results page.

The following tests FAILED:
    121 - PID_TEST.py (Failed)

• https://build.osrfoundation.org/job/ignition_math-ci-pr_any-ubuntu_auto-amd64/1318/testReport/(root)/

[scpeters]

The following stack overflow link indicates that pytest can be used with the --junitxml flag to generate JUNIT result files.

• https://stackoverflow.com/a/19126329

The ament_has_pytest.cmake and ament_add_pytest_test.cmake files may be a useful reference. Perhaps that functionality should be added to ign-cmake?