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/cfg/traps_early.cton

@@ -4,7 +4,7 @@ test print-cfg
 test verifier
 
 function %nonsense(i32) {
-; check: digraph %nonsense {
+; check: digraph "%nonsense" {
 
 ebb0(v1: i32):
     trap user0      ; error: terminator instruction was encountered before the end