Use `TableTest` to cut down boiler plate in tests

[dantownsend]

Almost all of our tests are like this:

class MyTest(TestCase):
    def setUp(self):
        # create tables

    def tearDown(self):
        # drop tables

Recently (when adding more SQL functions to Piccolo) we added a sub class of TestCase called TableTest, which cuts down the boiler plate of creating / dropping tables.

class MyTest(TableTest):  # The parent class creates / drops the tables for us
    tables = [MyTable]

We should move some of our existing tests over to use TableTest to remove boiler plate from the code base.