shims: fs: silence stderr instead of stdout.

Fixes #2143

src/shims/posix/fs.rs

@@ -307,14 +307,14 @@ pub struct FileHandler {
 impl<'tcx> FileHandler {
     pub(crate) fn new(mute_stdout_stderr: bool) -> FileHandler {
         let mut handles: BTreeMap<_, Box<dyn FileDescriptor>> = BTreeMap::new();
+        handles.insert(0i32, Box::new(io::stdin()));
         if mute_stdout_stderr {
-            handles.insert(0i32, Box::new(DummyOutput));
             handles.insert(1i32, Box::new(DummyOutput));
+            handles.insert(2i32, Box::new(DummyOutput));
         } else {
-            handles.insert(0i32, Box::new(io::stdin()));
             handles.insert(1i32, Box::new(io::stdout()));
+            handles.insert(2i32, Box::new(io::stderr()));
         }
-        handles.insert(2i32, Box::new(io::stderr()));
         FileHandler { handles }
     }
 

tests/run-pass/hide_stdout.rs

@@ -1,5 +1,6 @@
 // compile-flags: -Zmiri-mute-stdout-stderr
 
 fn main() {
-    println!("cake");
+    println!("print to stdout");
+    eprintln!("print to stderr");
 }