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/not_connected_error.py

@@ -0,0 +1,13 @@
+from scenarios import *
+
+from scenarios.simple_channel_join import expect_self_join_presence
+
+scenario = (
+    sequences.handshake(),
+    send_stanza("<presence type='unavailable' from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}/{nick_one}' />"),
+    # Fixme: what is the purpose of this test? Check that we don’t receive anything here…?
+
+    send_stanza("<presence from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}/{nick_one}' />"),
+    sequences.connection(),
+    expect_self_join_presence(jid='{jid_one}/{resource_one}', chan="#foo", nick="{nick_one}"),
+)