Subclass from congress.tests.base.TestCase

Previously, our tests were inconsistent in their superclass; some subclassed
from congress.tests.base.TestCase, while others subclassed from
unittest.TestCase.

Our tests need to subclass from congress.tests.base.TestCase so that logging
infrastructure and other things are initialized.  This is why some tests would
not produce any log messages when the test failed.

All future tests should subclass from congress.tests.base.TestCase.

This change alters our use of assertRaises() since the congress.tests.base
class's implementation of assertRaises() does not support the "with" syntax.

Change-Id: Ic49cb90b462fe37ba19a9431f7057eac55b41ada
Merge "Subclass from congress.tests.base.TestCase"