Add tests and descriptions for the functions in json_util.c

The user might want to use the userdata for something different, so the
serializer should ignore it by default.

Explicitly setting the serializer to json_object_double_to_json_string will
still make it interpret the userdata as a format string.

Do some basic checks on ints, doubles, strings, arrays and "complex" objects.

Conflicts:
	json_util.c

This is particularly useful when replacing values in a loop, since it allows
 the key used by json_object_object_foreach to continue to be used.

Extend the run_output_test() function so we actually can pass command line
 parameters and so we can support different output files for the same test
 executable.
Also provide some hints about what to do if a test fails (i.e. set VERBOSE=1).