'Enable a plugin with acyclic dependencies.'

def test_enable_dependencies(self):

    p = PluginManager(None)
    events = []
    A.plugin_init = (lambda s: events.append('init_a'))
    B.plugin_init = (lambda s: events.append('init_b'))
    p.enable('f')
    self.assertEqual(len(p), 3, 'Wrong    number    of    enabled    plugins.')
    self.assertTrue(('init_a' in events), 'Dependency    A    not    enabled.')
    self.assertTrue(('init_b' in events), 'Dependency    B    not    enabled.')
