cp: look for specific error messages in tests

tests/by-util/test_cp.rs

@@ -170,25 +170,35 @@ fn test_cp_multiple_files() {
 
 #[test]
 fn test_cp_multiple_files_with_nonexistent_file() {
+    #[cfg(windows)]
+    let error_msg = "The system cannot find the file specified";
+    #[cfg(not(windows))]
+    let error_msg = format!("'{TEST_NONEXISTENT_FILE}': No such file or directory");
     let (at, mut ucmd) = at_and_ucmd!();
     ucmd.arg(TEST_HELLO_WORLD_SOURCE)
         .arg(TEST_NONEXISTENT_FILE)
         .arg(TEST_HOW_ARE_YOU_SOURCE)
         .arg(TEST_COPY_TO_FOLDER)
-        .fails();
+        .fails()
+        .stderr_contains(error_msg);
 
     assert_eq!(at.read(TEST_COPY_TO_FOLDER_FILE), "Hello, World!\n");
     assert_eq!(at.read(TEST_HOW_ARE_YOU_DEST), "How are you?\n");
 }
 
 #[test]
 fn test_cp_multiple_files_with_empty_file_name() {
+    #[cfg(windows)]
+    let error_msg = "The system cannot find the path specified";
+    #[cfg(not(windows))]
+    let error_msg = "'': No such file or directory";
     let (at, mut ucmd) = at_and_ucmd!();
     ucmd.arg(TEST_HELLO_WORLD_SOURCE)
         .arg("")
         .arg(TEST_HOW_ARE_YOU_SOURCE)
         .arg(TEST_COPY_TO_FOLDER)
-        .fails();
+        .fails()
+        .stderr_contains(error_msg);
 
     assert_eq!(at.read(TEST_COPY_TO_FOLDER_FILE), "Hello, World!\n");
     assert_eq!(at.read(TEST_HOW_ARE_YOU_DEST), "How are you?\n");