Add rudimentary unit tests for JValue.cpp

Also removes NON_NULL from jvalue_copy and currently the NON_NULL in the
function definition causes the optimizer to remove all NULL checks from
that function, causing NULL dereferencing in worst case scenarios

[GF-6132] libpbnjson: Create unit tests for JValue

Open-webOS-DCO-1.0-Signed-off-by: Janne Hurskainen <janne.hurskainen@lge.com>

Change-Id: If0d78f995ff33d1e2b7e3247701a2ca8d69fa99d