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.
This commit was originally from @fredfishgames, but it
needed a big rebase due to use letting it sit too long.
Also, we decided not to have shebangs at the top of test
files.
