Update test scripts for node addition and deletion

Update relevant test scripts to better demonstrate how the oBIX server
supports nodes addition and deletion for the time being. In particular,
changes of the val attribute require the target node writable, subnode
deletion require either the target node or its direct parent writable,
but addition of new children won't require the target node writable.
Merge pull request #29 from Qingtao-Cao/devel

Update test scripts for node addition and deletion