Add doc on expected test file regeneration

Using the new SCANCODE_REGEN_TEST_FIXTURES

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

docs/source/contribute/contrib_dev.rst

@@ -109,7 +109,30 @@ These are enabled by adding a ``--test-suite`` option to the pytest command.
   extensive data-driven and data validations (for package, copyright and license
   detection)
 
+In some cases we need to regenerate test data when expected behavious/result data
+structures change, and we have an environement variable to regenerate test data.
+`SCANCODE_REGEN_TEST_FIXTURES` is present in `scancode_config` and this can be
+set to regenerate test data for specific tests like this:
 
+``SCANCODE_REGEN_TEST_FIXTURES=yes pytest -vvs tests/packagedcode/test_package_models.py``
+
+This command will only regenerate test data for only the tests in `test_package_models.py`,
+and we can further specify the tests to regen by using more pytest options like `--lf` and
+`-k test_instances`.
+
+If test data is regenerated, it is important to review the diff for test files and
+carefully go through all of it to make sure there are no unintended changes there,
+and then commit all the regenerated test data.
+
+To help debug in scancode, we use logging. There are different environement variables
+you need to set to turn on logging. In packagedcode::
+
+``SCANCODE_DEBUG_PACKAGE=yes pytest -vvs tests/packagedcode/ --lf``
+
+Or set the ``TRACE`` variable to ``True``. This enables ``logger_debug`` functions
+logging variables and shows code execution paths by logging and printing the logs
+in the terminal. If debugging full scans run by click, you have to raise exceptions
+in addition to setting the TRACE to enable logging.
 
 .. _scancode_toolkit_development_thirdparty_libraries: