Tests: fix unload_plugins

Don't mutate the base classes. This was causing plugins to break on the second
test that uses them, since after they were unloaded their class-level fields
were broken (since the modules are not re-loaded).

This makes it more clear than ever that we need to encapsulate plugin loading
state using some manner of PluginManager.

test/helper.py

@@ -202,8 +202,6 @@ class TestHelper(object):
         """
         # FIXME this should eventually be handled by a plugin manager
         beets.config['plugins'] = []
-        for plugin in beets.plugins._classes:
-            plugin.listeners = None
         beets.plugins._classes = set()
         beets.plugins._instances = {}
         Item._types = {}

test/test_permissions.py

@@ -15,6 +15,7 @@ class PermissionsPluginTest(unittest.TestCase, TestHelper):
 
     def tearDown(self):
         self.teardown_beets()
+        self.unload_plugins()
 
     def test_permissions_on_album_imported(self):
         self.importer = self.create_importer()