Add test for keyword "extends" in JSON schema

:Release Notes:
Add a new test to check behaviour of json validator on extended schema

:Detailed Notes:
libpbnjson does not contain tests for json schema validator that will check all legal schema keywords
This commit add such test. For now it will check only "extends" keyword

:Testing Performed:
A new test was added

:QA Notes:

:Issues Addressed:
[GF-4639] pbnjson "$ref" and "extends" keywords not working

Open-webOS-DCO-1.0-Signed-off-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>

Change-Id: Ia36425a35f6cbe86a8b86d4aac24004f5b51a23c
Reviewed-on: https://g2g.palm.com/2838
Reviewed-by: Build Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>