Why not just instantiate a TestCase instance and use that?

>>> from unittest import TestCase
>>> t = TestCase()
>>> t.assertEqual('foo', 'bar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/compile/py3k-cpython/Lib/unittest/case.py", line 642, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/compile/py3k-cpython/Lib/unittest/case.py", line 1021, in assertMultiLineEqual
    self.fail(self._formatMessage(msg, standardMsg))
  File "/compile/py3k-cpython/Lib/unittest/case.py", line 509, in fail
    raise self.failureException(msg)
AssertionError: 'foo' != 'bar'
- foo
+ bar