Add ignore-pretty to the fn signature verification test

Author: blackbeam

src/test/run-pass/test-fn-signature-verification-for-explicit-return-type.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // compile-flags: --test
+// ignore-pretty: does not work well with `--test`
 extern crate test;
 
 #[bench]