e2e: Implicitely include the handshake in all tests

Instead of having to write it manually everytime

tests/end_to_end/scenarios/execute_ping_adhoc_command.py

@@ -1,7 +1,6 @@
 from scenarios import *
 
 scenario = (
-    sequences.handshake(),
     send_stanza("<iq type='set' id='ping-command1' from='{jid_one}/{resource_one}' to='{biboumi_host}'><command xmlns='http://jabber.org/protocol/commands' node='ping' action='execute' /></iq>"),
     expect_stanza("/iq[@type='result']/commands:command[@node='ping'][@status='completed']/commands:note[@type='info'][text()='Pong']")
 )