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/__init__.py

@@ -0,0 +1,10 @@
+# Do "from scenarios import *" instead of repeating these imports everytime in every scenario
+
+from functions import expect_stanza, send_stanza, expect_unordered, save_value, extract_attribute, extract_text, sleep_for, save_current_timestamp_plus_delta
+import datetime
+import sequences
+import scenarios.simple_channel_join
+import scenarios.channel_join_with_two_users
+import scenarios.simple_channel_join_fixed
+import scenarios.channel_join_on_fixed_irc_server
+import scenarios.multiple_channels_join