Use for_label_no_replacement.

This will cause certain unsupported legacy encodings to act as if they
don't exist, in order to avoid using an unhelpful (in the context of
file searching) "replacement" encoding.

Kudos to @hsivonen for chirping about this!

tests/workdir.rs

@@ -256,6 +256,23 @@ impl WorkDir {
                    String::from_utf8_lossy(&o.stderr));
         }
     }
+
+    /// Runs the given command and asserts that something was printed to
+    /// stderr.
+    pub fn assert_non_empty_stderr(&self, cmd: &mut process::Command) {
+        let o = cmd.output().unwrap();
+        if o.status.success() || o.stderr.is_empty() {
+            panic!("\n\n===== {:?} =====\n\
+                    command succeeded but expected failure!\
+                    \n\ncwd: {}\
+                    \n\nstatus: {}\
+                    \n\nstdout: {}\n\nstderr: {}\
+                    \n\n=====\n",
+                   cmd, self.dir.display(), o.status,
+                   String::from_utf8_lossy(&o.stdout),
+                   String::from_utf8_lossy(&o.stderr));
+        }
+    }
 }
 
 fn nice_err<P: AsRef<Path>, T, E: error::Error>(