backend/headless: actually perform output test

Currently the headless backend does not actually implement the
wlr_output_test function, causing tests containing output state
unsupported by the headless backend to succeed while committing the same
state will always fail.

This commit fixes that by actually hooking up the already exisiting test
function.

References: https://codeberg.org/river/river/issues/1154

backend/headless/output.c

@@ -89,6 +89,7 @@ static void output_destroy(struct wlr_output *wlr_output) {
 
 static const struct wlr_output_impl output_impl = {
 	.destroy = output_destroy,
+	.test = output_test,
 	.commit = output_commit,
 };