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/loops2.cton

@@ -29,3 +29,29 @@ function %test(i32) {
     ebb9:
         return
 }
+
+function %test(i32) native {
+    ebb0(v0: i32):
+        brz v0, ebb1    ; dominates: ebb1 ebb3 ebb4 ebb5
+        jump ebb2       ; dominates: ebb2
+    ebb1:
+        jump ebb3
+    ebb2:
+        brz v0, ebb4
+        jump ebb5
+    ebb3:
+        jump ebb4
+    ebb4:
+        brz v0, ebb3
+        jump ebb5
+    ebb5:
+        brz v0, ebb4
+        return
+}
+; check: cfg_postorder:
+; sameln: ebb5
+; sameln: ebb3
+; sameln: ebb4
+; sameln: ebb2
+; sameln: ebb1
+; sameln: ebb0