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/list_adhoc.py

@@ -0,0 +1,11 @@
+from scenarios import *
+
+scenario = (
+    sequences.handshake(),
+    send_stanza("<iq type='get' id='idwhatever' from='{jid_one}/{resource_one}' to='{biboumi_host}'><query xmlns='http://jabber.org/protocol/disco#items' node='http://jabber.org/protocol/commands' /></iq>"),
+    expect_stanza("/iq[@type='result']/disco_items:query[@node='http://jabber.org/protocol/commands']",
+                  "/iq/disco_items:query/disco_items:item[@node='configure']",
+                  "/iq/disco_items:query/disco_items:item[4]",
+                  "!/iq/disco_items:query/disco_items:item[5]"),
+
+)