fix a bug preventing EM_READY being applied

The lines in _update_superevent

if 'preferred_event' in kwargs and is_complete(new_event_dict):
    gracedb.create_label('EM_READY', superevent_id)

is not the correct logic since a preferred event may exists, but it maybe incomplete. A label applied later might make it complete.

The correct logic is

if is_complete(new_event_dict):
    gracedb.create_label('EM_READY', superevent_id)

This is because new_event_dictionary completeness is necessary and sufficient to conclude that the preferred event is complete.

• Either the new event has become the preferred event
• Or the existing preferred event is already a more significant complete event. In this case EM_READY would already have been applied.

The unittests now capture the different possible scenarios.