Prepare bzrlib.plugin to use the new test loader.
* bzrlib/tests/test_plugins.py:
(TestPlugins.test_no_load_tests_gives_None_for_load_tests): Smoke
tests for plugin.load_tests().
* bzrlib/plugin.py:
(PlugIn.load_tests): Add the ability to use load_tests() instead
of test_suite().