e2e: Implicitely include the handshake in all tests

Instead of having to write it manually everytime

tests/end_to_end/scenarios/basic_handshake_success.py

@@ -1,5 +1,8 @@
 from scenarios import *
 
+# At the start of every scenario, we automatically insert a
+# sequences.handshake() call. So, this scenario is just here to test that
+# this basic thing works fine.
+
 scenario = (
-    sequences.handshake(),
 )