Split all the e2e tests in their own files

Some duplication is avoided, but most importantly: it’s much much cleaner,
easier to find, functions have been cleaned up (functools.partial usages are
now hidden), etc.

tests/end_to_end/scenarios/execute_ping_adhoc_command.py

@@ -0,0 +1,7 @@
+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']")
+)