'Enable a single plugin.'

def test_enable(self):

    p = PluginManager(None)
    events = []
    def init(self):
        events.append('init')
    A.plugin_init = init
    p.enable('a')
    self.assertEqual(len(p), 1, 'Wrong    number    of    enabled    plugins.')
    self.assertEqual(events, ['init'], 'Plugin    init    method    not    called.')
