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