Make json_object_object_add() indicate success or failure, test fix

json_object_object_add() now indicates success or failure via its return value. The previous behaviour, namely to exit the program on failure, is cumbersome in large-scale applications.

Note that the change of return value from void to int may break the ABI so that programs using json-c may have to be rebuilt.

Unrelated to this change, I found I had to adjust the expected output of test_parse for a successful build.