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/muc_traffic_info.py

@@ -0,0 +1,8 @@
+from scenarios import *
+
+scenario = (
+    sequences.handshake(),
+
+    send_stanza("<iq from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}' id='1' type='get'><query xmlns='http://jabber.org/protocol/disco#info' node='http://jabber.org/protocol/muc#traffic'/></iq>"),
+    expect_stanza("/iq[@from='#foo%{irc_server_one}'][@to='{jid_one}/{resource_one}'][@type='result']/disco_info:query[@node='http://jabber.org/protocol/muc#traffic']"),
+)