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

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

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.

* add test NnToJsonLeafrefType to nn/to/json/test
  * positive tests of leafref
    * absolut path
    * relative path
    * leafref to not leaf
    * from leaflist to leaf
    * from leafref to leafref
  * negative tests of leafref
    * non existing Leaf

Change-Id: I652204071d874d6c7a2a65e100dd68c16a12af8f
Signed-off-by: Jakub Toth <jatoth@cisco.com>
(cherry picked from commit 1f4b4062cbdd0cfadcf61e60bce9ee9e951c975c)