Convert the CFG traversal tests to file tests.

Add a "cfg_postorder:" printout to the "test domtree" file tests and use
that to check the computed CFG post-order instead of doing it manually
with Rust code.

cranelift/filetests/domtree/basic.cton

@@ -11,3 +11,8 @@ function %test(i32) {
     ebb3:
         return
 }
+; check: cfg_postorder:
+; sameln: ebb2
+; sameln: ebb3
+; sameln: ebb1
+; sameln: ebb0