Add tests for XDS protocol support

Add tests for XDS protocol support

* test/lisp/x-dnd-tests.el (x-dnd-tests-xds-property-value): New
variable.
(x-window-property): Handle new kind of window property.
(x-dnd-tests-xds-target-dir, x-dnd-tests-xds-name)
(x-dnd-tests-xds-include-hostname): New variables.
(x-dnd-tests-call-xds-converter): New function.
(x-begin-drag, x-change-window-property):
(x-delete-window-property): New replacement functions.
(x-dnd-tests-do-direct-save-internal): New function.
(x-dnd-tests-do-direct-save): New test.