Refactor tests

[aaugustin]

I'd like to get rid of the runtests.py script, and be able to run the tests with just DJANGO_SETTINGS_MODULE=xxx django-admin.py test. For an example, see https://github.com/aaugustin/django-sesame. (NB: putting the test settings inside the same package as the tests is known to cause import order issues with at least some versions of Django.)

I also suggest to split the tests across several modules:

• general tests for the toolbar (show/hide, insertion in the response, etc.)
• specific tests for each panel (one module per panel)
• tests for the debugsqlshell command