Add test for all groups have a coordinator.

Some ZoneGroups have no coordinator set.
Amend zgs fixture for first group coordinator

The first ZoneGroup in the ZGS fixture nominated as coordinator a UUID not
present in that group.
Fix zone group built from zgs without coordinator.

Setting group coordinator to None inside the loop over group members cancels
a previous setting of the coordinator.
Merge pull request #230 from simonalpha/fix-missing-zg-coordinator

Fix ZoneGroups built from ZoneGroupState without coordinator