Update pytest how to for BIND system tests: add fixture reference

pytest-howto.md

@@ -1,10 +1,14 @@
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 title: pytest how-to for BIND system tests
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+
 See [[/bin/tests/system/README|/../blob/main/bin/tests/system/README.md]] first
 
 # Debugging
 - See "Test Artifacts" chapter in README first, for real!
 - If test hangs on startup or exits one minute after start - most probably servers did not start. That's typically a config error.
   - `pytest --log-level DEBUG` should show which server does not respond to TCP queries
-- See also `pytest.log.txt` in directory with artifacts.
+- See also `pytest.log.txt` in directory with artifacts.
+
+# Magic variables
+Pytest has framework for providing 'context' to tests in a consistent manner. Their term for it is 'fixture' and it manifests itself as a function parameter with name which is nowhere to be found. See https://docs.pytest.org/en/7.1.x/explanation/fixtures.html