hello: also ensure stderr works

Author: RalfJung

src/tools/miri/tests/pass/hello.rs

@@ -1,3 +1,4 @@
 fn main() {
     println!("Hello, world!");
+    eprintln!("Hello, error!");
 }

src/tools/miri/tests/pass/hello.stderr

@@ -0,0 +1 @@
+Hello, error!