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 CnSnToXmlNotExistingLeafTypeTest test
* add NnToXmlNotExistingLeafTypeTest to nn/to/xml/test/
  * negative test for not existing leaf type

Change-Id: I6f55fd82c40a03726aecb78cfc2ece9888cf9717
Signed-off-by: Jakub Toth <jatoth@cisco.com>
(cherry picked from commit 0c957832fd6587f5cfe6fb8de215d97cd6fd9c24)