e2e: Implicitely include the handshake in all tests

Instead of having to write it manually everytime

tests/end_to_end/scenarios/self_disco_info.py

@@ -1,7 +1,6 @@
 from scenarios import *
 
 scenario = (
-    sequences.handshake(),
     send_stanza("<iq type='get' id='get1' from='{jid_one}/{resource_one}' to='{biboumi_host}'><query xmlns='http://jabber.org/protocol/disco#info'/></iq>"),
     expect_stanza("/iq[@type='result']/disco_info:query/disco_info:identity[@category='conference'][@type='irc'][@name='Biboumi XMPP-IRC gateway']",
                   "/iq/disco_info:query/disco_info:feature[@var='jabber:iq:version']",