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/loop.cton

@@ -3,7 +3,7 @@ test print-cfg
 test verifier
 
 function %nonsense(i32, i32) -> f32 {
-; check: digraph %nonsense {
+; check: digraph "%nonsense" {
 ; regex: I=\binst\d+\b
 ; check: label="{ebb0 | <$(BRZ=$I)>brz ebb2 | <$(JUMP=$I)>jump ebb1}"]
 

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

cranelift/filetests/cfg/unused_node.cton

@@ -2,7 +2,7 @@
 test print-cfg
 
 function %not_reached(i32) -> i32 {
-; check: digraph %not_reached {
+; check: digraph "%not_reached" {
 ; check:     ebb0 [shape=record, label="{ebb0 | <inst0>brnz ebb2}"]
 ; check:     ebb1 [shape=record, label="{ebb1 | <inst4>jump ebb0}"]
 ; check:     ebb2 [shape=record, label="{ebb2}"]