Convert the DFG tests to use filecheck.

tests/cfg/unused_node.cton

@@ -1,15 +1,19 @@
 ; For testing cfg generation where some block is never reached.
 
 function not_reached(i32) -> i32 {
+; 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}"]
 
 ebb0(v0: i32):
-    brnz v0, ebb2       ;;;; ebb0:inst0 -> ebb2
+    brnz v0, ebb2       ; unordered: ebb0:inst0 -> ebb2
     trap
 
 ebb1:
     v1 = iconst.i32 1
     v2 = iadd v0, v1
-    jump ebb0(v2)       ;;;; ebb1:inst4 -> ebb0
+    jump ebb0(v2)       ; unordered: ebb1:inst4 -> ebb0
 
 ebb2:
     return v0