More tests for leaves during a partial state resync (#617)

* Unskip leave-during-resync test
* Group leave-during-resync tests under supertest
* Test that the original joiner can leave if someone else arrives.
* Test you can rejoin even if resync didn't complete
* Test someone else can join after you leave
* Test that remote kicks work during resync
* Test that remote bans work during resync, and you can't rejoin during resync