Add test for extended object creation/deletion.

test/demo/main.gd

@@ -10,6 +10,7 @@ func _ready():
 	($Example as Example).simple_const_func() # Force use of ptrcall
 	prints("returned", $Example.return_something("some string"))
 	prints("returned const", $Example.return_something_const())
+	prints("returned ref", $Example.return_extended_ref())
 	prints("vararg args", $Example.varargs_func("some", "arguments", "to", "test"))
 
 	# Use properties.