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/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
+}