Add a 'test verifier' sub-test.

This test runs the verifier on each function and matches the resulting
verifier error against the "error:" annotation.

Move the existing verifier test into filetests/verifier/ and use the new
syntex.

cranelift/filetests/cfg/loop.cton

@@ -1,5 +1,6 @@
 ; For testing cfg generation. This code is nonsense.
 test print-cfg
+test verifier
 
 function nonsense(i32, i32) -> f32 {
 ; check: digraph nonsense {

cranelift/filetests/cfg/traps_early.cton

@@ -1,12 +1,13 @@
 ; For testing cfg generation. This code explores the implications of encountering
 ; a terminating instruction before any connections have been made.
 test print-cfg
+test verifier
 
 function nonsense(i32) {
 ; check: digraph nonsense {
 
 ebb0(v1: i32):
-    trap
+    trap            ; error: terminator instruction was encountered before the end
     brnz v1, ebb2   ; unordered: ebb0:inst1 -> ebb2
     jump ebb1       ; unordered: ebb0:inst2 -> ebb1
 

cranelift/filetests/verifier/bad_layout.cton

@@ -0,0 +1,19 @@
+test verifier
+
+function test(i32) {
+    ebb0(v0: i32):
+        jump ebb1       ; error: terminator
+        return
+    ebb1:
+        jump ebb2
+        brz v0, ebb3
+    ebb2:
+        jump ebb3
+    ebb3:
+        return
+}
+
+function test(i32) {    ; Ok
+    ebb0(v0: i32):
+        return
+}