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/self_invite.py

@@ -0,0 +1,7 @@
+from scenarios import *
+
+scenario = (
+    scenarios.simple_channel_join.scenario,
+    send_stanza("<message from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}'><x xmlns='http://jabber.org/protocol/muc#user'><invite to='{nick_one}'/></x></message>"),
+    expect_stanza("/message/body[text()='{nick_one} is already on channel #foo']")
+)