Muc multi session

Tests for multisession esl/MongooseIM#240 implemented in esl/MongooseIM#246
+1
Also, the `_helper` bit isn't really needed in my opinion.
Generally, this entire `helper` is made up of duplicated code, like the bit below:
```erlang
 Presence = escalus:wait_for_stanza(Bob),
 is_presence_with_affiliation(Presence, <<"none">>),
 is_self_presence(Bob, ?config(room, Config), Presence),

 is_subject_message(escalus:wait_for_stanza(Bob)), 
``` 
, which could be extracted, named for what it does, and used in the top-level iterator, as suggested above by @fenek.

Also, the name `is_message_correct` doesn't tell the reader whether this is an assertion or not. To me, it looks like an ordinary predicate, therefore CT won't complain if it returns `false`.

You can either say
```
true = is_message_correct(Something, Something, [DarkSide]),
```
**or**
```
assert_message_is_correct(Something, Something, [DarkSide]),
```