[ci] Code coverage integration test

[daguirre11]

Summary

As part of the contributions made in #7031, I would like to add a CI step that runs code coverage on the Python package tests with a minimum threshold for passing. I would defer to the repo owners on the exact threshold, but suggest at least 90%, preferably 95%.

Motivation

This will ensure that each feature is adequately tested, new features include test coverage, and existing features continue to work as expected.

Description

Something along the lines of:

• Add code_cov.yaml CI workflow
• Add a job to code_cov.yaml that installs dependencies and runs the Python package tests
• Add a step to code_cov.yaml:

  run: |
    coverage erase
    pytest --cov=lightgbm --cov-branch --cov-report=term-missing --cov-report=html --cov-report=xml --junitxml=junit/test-results.xml -vv tests/python_package_test/
    coverage report --fail-under=90
  env:
    PY_COLORS: 1

References

#7031

[jameslamb]

Thanks for your interest and for all of your continued help with the test coverage here!

I do not support adding codecov here... I want to limit the number of third-party applications as much as possible, because each of them brings in new maintenance burden, security risk, and risk to CI stability.

BUT... I'd welcome a PR that:

• adds pytest-cov as a testing dependency
• generates coverage reports printed to the logs in existing Python CI jobs
• enforces a lower threshold (though based on what I see in [python-package] expand test coverage #7031 it should be more like 80%)

If you're interested in doing that, please go ahead and open a PR. We can talk more about specifics and I can help answer your questions there.