Fix missing initializer warning in gobject/tests/ifaceproperties.c:test_iface_get_type()

gobject/tests/ifaceproperties.c: In function ‘test_iface_get_type’:
gobject/tests/ifaceproperties.c:144:1: error: missing initializer for field ‘class_finalize’ of ‘GTypeInfo’ {aka ‘const struct _GTypeInfo’}
  144 | static DEFINE_IFACE (TestIface, test_iface, NULL, test_iface_default_init)
      | ^~~~~~
In file included from gobject/gobject.h:24,
                 from gobject/gbinding.h:29,
                 from glib/glib-object.h:22,
                 from gobject/tests/ifaceproperties.c:21:
gobject/gtype.h:1054:26: note: ‘class_finalize’ declared here
 1054 |   GClassFinalizeFunc     class_finalize;
      |                          ^~~~~~~~~~~~~~

gobject/tests/testcommon.h

@@ -72,6 +72,12 @@ prefix ## _get_type (void)					\
 	(GBaseInitFunc)	base_init,				\
 	(GBaseFinalizeFunc) NULL,				\
 	(GClassInitFunc) dflt_init,				\
+	(GClassFinalizeFunc) NULL,				\
+	(gconstpointer) NULL,					\
+	(guint16) 0,						\
+	(guint16) 0,						\
+	(GInstanceInitFunc) NULL,				\
+	(const GTypeValueTable*) NULL,				\
       };							\
 								\
       iface_type = g_type_register_static (G_TYPE_INTERFACE,	\