MB-23535: Update CMD_OBSERVE tests for Ephemeral buckets

MB-23535: Update CMD_OBSERVE tests for Ephemeral buckets

CMD_OBSERVE is still valid for Ephemeral buckets, with two caveats:

1. It will never return a value of OBS_STATE_PERSISTED for the
   `persisted` field - instead it will return OBS_STATE_NOT_PERSISTED
   for non-deleted, valid items.

2. Items which have been deleted but not yet purged with return
   OBS_STATE_LOGICAL_DEL instead of OBS_STATE_NOT_FOUND.

As such, re-enable the existing CMD_OBSERVE tests which were
previously disabled for Ephemeral buckets, and update based on the two
above differences.

Change-Id: I3a44531abf7cfd0bfad8fbd89a76f4cad94133fb
Reviewed-on: http://review.couchbase.org/76966
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>