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/quit.py

@@ -0,0 +1,12 @@
+from scenarios import *
+
+import scenarios.simple_channel_join
+
+scenario = (
+    scenarios.simple_channel_join.scenario,
+
+    # Send a raw QUIT message
+    send_stanza("<message from='{jid_one}/{resource_one}' to='{irc_server_one}' type='chat'><body>QUIT bye bye</body></message>"),
+    expect_stanza("/presence[@from='#foo%{irc_server_one}/{nick_one}'][@type='unavailable']/muc_user:x/muc_user:status[@code='110']"),
+)
+