e2e: Implicitely include the handshake in all tests

Instead of having to write it manually everytime

tests/end_to_end/scenarios/not_connected_error.py

@@ -3,7 +3,6 @@ from scenarios import *
 from scenarios.simple_channel_join import expect_self_join_presence
 
 scenario = (
-    sequences.handshake(),
     send_stanza("<presence type='unavailable' from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}/{nick_one}' />"),
     # Fixme: what is the purpose of this test? Check that we don’t receive anything here…?