Testdoc generates docs for TestCases that are imported

For example,

    from bzrlib.tests import TestCase

    class TestMyCode(TestCase):
        ...

will cause TestCase to be documented (in addition to TestMyCode), even though it's not defined in this file.

A workaround is to just import the module:

    from bzrlib import tests

    class TestMyCode(tests.TestCase):
        ...