test_inotify: do not assume watch descriptor values

It is not safe to assume the values returned by add_watch(), so we add a local helper get_wd() to retrieve them. This fixes a problem in TestInotifyTree.test__cycle() where the wd's for the 'aa' and 'bb' paths are not '2' and '3', respectively.

A second issue is that Inotify._get_event_names() should return a set or sorted list to avoid comparison problems, but that is not addressed here since it could be viewed as changing the API.