Since the property refactor, write-only properties have not been working.
The problem was that a getter and setter function were not defined for
them, because is_gobject_property_name() did not consider them to be
properties. Now, the setter function works as normal while the getter
function just pretends a write-only property has the value of undefined.

The test is marked pending until a test is added to the
gobject-introspection test suite.