Unit test reorganization.

Moved SleekTest to sleekxmpp.test.

Organized test suites by their focus.
- Suites focused on testing stanza objects are named test_stanza_X.py
- Suites focused on testing stream behavior are name test_stream_X.py

tests/test_events.py

@@ -1,6 +1,5 @@
-import sleekxmpp
 import time
-from . sleektest import *
+from sleekxmpp.test import *
 
 
 class TestEvents(SleekTest):