BUG 1440 - additional tests for data-codec-gson

Tests of transformation:
- json stream -> normalized node structure -
  JsonStreamToNormalizedNodeTest
- normalized node structure -> json stream -
  NormalizedNodeToJsonStreamTest
were added

Change-Id: Ieff2e565e776e5193219b07f6e0ab98165402237
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>