Previously the test-bots script filtered out base-class tests from
BotTestCase. With this change, BotTestCase continues to inherit from
unittest.TestCase, but the default test_* methods previously in this
class are now in a new DefaultTests class, which does not. Instead, each
bot needs to inherit from BotTestCase and DefaultTests *explicitly*.
This avoids the need to filter out the base-class tests, which
simplifies the test-bots script, and may ease any migration to eg.
pytest.
The DefaultTests class does require some non-implemented methods which
BotTestCase provides.
