tests: README.md caution about pytest run in GitHub Actions

Author: cclauss

tests/README.md

@@ -0,0 +1,6 @@
+> [!CAUTION]
+> `pytest tests/` will be run in GitHub Actions.
+
+All Python files in this directory that meet [pytest discovery rules](https://docs.pytest.org/en/stable/explanation/goodpractices.html#conventions-for-python-test-discovery) will be examined by pytest and all tests that it finds will be run.
+
+Files of the form `test_*.py` or `*_test.py` should avoid code at global scope because it will be executed in pytest's discovery process.  Classes, functions, and code in `if __name__ == "__main__":` blocks will not be run in this discovery process, but code at global scope (including imports) will.