Bug 2358 - Remove tests cnsn to xml and add tests nn to xml

Tests of codecs are included in codecs yangtools yang-data-codec-gson and yang-data-impl but
we test codec on input data and also with presentation of Rest path.

* remove CnSnInstanceIdentifierToXmlTest test
* add NnInstanceIdentifierToXmlTest to nn/to/xml/test/
  * positive test for instance identifier

Change-Id: Iceb73ed4d96f7686fca04047524716532297a2f8
Signed-off-by: Jakub Toth <jatoth@cisco.com>
(cherry picked from commit 6c0c438071d2e85ef2f08070935cb310165f25cf)