Add tests for incoming calls in group calls (#2597)
* Add tests for incoming calls in group calls
Inspiration wwlecome for the renamed describe group which we're
really abusing for a bunch of things that happen to have the same
dependencies.
Fixes https://github.com/vector-im/element-call/issues/532
* Extract incoming call tests out into their own describe
and get the lexicographical ordering to match who should be calling who
* Trailing space
