e2e: Implicitely include the handshake in all tests

Instead of having to write it manually everytime

tests/end_to_end/scenarios/fixed_irc_server_subscription.py

@@ -3,7 +3,6 @@ from scenarios import *
 conf = 'fixed_server'
 
 scenario = (
-    sequences.handshake(),
     send_stanza("<presence type='subscribe' from='{jid_one}/{resource_one}' to='{biboumi_host}' id='sub1' />"),
     expect_stanza("/presence[@to='{jid_one}'][@from='{biboumi_host}'][@type='subscribed']")
 )