Add more README documentation on how existing test drivers are structured.

Add more README documentation on how existing test drivers are structured.

More than once, I've reverse-engineered how the test suite uses test
drivers (app_driver.py and tool_driver.py) to invoke scripts within
the apps/ and traces/ directory. In order to save my future-self some
work in doing this yet again, I've written some documentation in
README.markdown in each directory.

Hopefully this documentation will be useful for anyone wanting to
write additional tests.