Muc multi session

Tests for multisession esl/MongooseIM#240 implemented in esl/MongooseIM#246
Ah, the hydra of many heads. Where is Hercules?

This is just a suggestion, but can't we say something like:
```erlang
init_per_testcase(Name, Config) where
  Name == some_test;
  Name == other_test;
  Name == yet another test ->
  something:dark_side(foo, Config).
```
That way we're adding 1 line per case name, and not a 3-line function head. This test suite needs cleaning anyway, so let's start.