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/src/tools/filetest/mod.rs

@@ -10,6 +10,7 @@ use filetest::runner::TestRunner;
 pub mod subtest;
 mod runner;
 mod domtree;
+mod verifier;
 
 /// Main entry point for `cton-util test`.
 ///