don't run pretty-rpass for tests using #![main]

Author: flaper87

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
+// no-pretty-expanded
 extern crate test;
 
 #[bench]