Add a unit test for the ValueNode class. The tests focus on the class
itself, without considering that is currently only used when parsing
YAML files. This duplicates some of the tests of the YamlParser class,
which will be dropped from the corresponding unit test in a subsequent
change.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Isaac Scott <isaac.scott@ideasonboard.com>
