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_ping_fixed_server.py

@@ -0,0 +1,11 @@
+from scenarios import *
+
+conf = "fixed_server"
+
+scenario = (
+    scenarios.simple_channel_join_fixed.scenario,
+    
+    # Send a ping to ourself
+    send_stanza("<iq type='get' from='{jid_one}/{resource_one}' id='first_ping' to='#foo@{biboumi_host}/{nick_one}'><ping xmlns='urn:xmpp:ping' /></iq>"),
+    expect_stanza("/iq[@from='#foo@{biboumi_host}/{nick_one}'][@type='result'][@to='{jid_one}/{resource_one}'][@id='first_ping']"),
+)