A basic unit test framework for ActivePapers

For a usage example, see

   code/python-packages/example_module
   code/run_tests

To use this framework in your own ActivePapers, link

   code/python-packages/unit_tests.py

into it. Then write test cases as in example_module, and a calclet
similar to run_tests for executing them.

The test log is produced as a standard ActivePaper item, with full
dependency tracking. If any code module has been changed, updating the
ActivePaper will automatically re-run the tests.