fmt: make test clearer

vlib/v/fmt/tests/anon_fn_as_param_keep.vv

@@ -1,4 +1,8 @@
-pub fn reduce(a []int, iter fn (int, int) int, accum_start int) int {
+struct Row{
+	id int
+}
+
+pub fn (a []Row) reduce(iter fn (int, int) int, accum_start int) int {
 	iter(accum_start)
 }