Ensure 'leave_group' result gotten before further work

Avoid the case where 'client3' doesn't leave the group before
we start to check for that removal in the main client by using
the get() operation on the 'client3' leave_group() operation.

Change-Id: I4f1cc3ea1088d0cd026023c72018ee6716b9bbc8
Merge "Ensure 'leave_group' result gotten before further work"