Clean up test framework module docstrings

[mwichmann]

Also a couple of typing additions and docstring changes in TestSCons. No functional changes.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[bdbaddog]

Is it worth adding the subclass of subclass of? If we generate API docs via Sphinx won't it explain that automaticaly?

[mwichmann]

Is it worth adding the subclass of subclass of? If we generate API docs via Sphinx won't it explain that automaticaly?

maybe not worth it... but we don't currently produce API docs of testfw.