Crude fix for bug #411413.
* bzrlib/tests/test_plugins.py:
(TestPluginMixin.create_plugin,
TestPluginMixin.create_plugin_package): Helpers to create simple
test plugins.
(TestDisablePlugin): Check that a disbled plugin can't be imported
nor automatically loaded.
* bzrlib/plugin.py:
(PluginBlackListImporter): Raise ImportError when a disabled plugin
is imported.