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/slash_me_channel_message.py

@@ -0,0 +1,18 @@
+from scenarios import *
+
+scenario = (
+    scenarios.channel_join_with_two_users.scenario,
+    # Send a channel message
+    send_stanza("<message from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}' type='groupchat'><body>/me rit en IRC</body></message>"),
+    # Receive the message, forwarded to the two users
+    expect_unordered(
+        [
+            "/message[@from='#foo%{irc_server_one}/{nick_one}'][@to='{jid_one}/{resource_one}'][@type='groupchat']/body[text()='/me rit en IRC']",
+            "/message/stable_id:stanza-id[@by='#foo%{irc_server_one}'][@id]"
+        ],
+        [
+            "/message[@from='#foo%{irc_server_one}/{nick_one}'][@to='{jid_two}/{resource_one}'][@type='groupchat']/body[text()='/me rit en IRC']",
+            "/message/stable_id:stanza-id[@by='#foo%{irc_server_one}'][@id]"
+        ],
+    ),
+)