test: fix tests

tests/by-util/test_stdbuf.rs

@@ -26,7 +26,7 @@ fn test_stdbuf_line_buffered_stdout() {
 #[test]
 fn test_stdbuf_no_buffer_option_fails() {
     new_ucmd!().args(&["head"]).fails().stderr_is(
-        "The following required arguments were not provided:\n    \
+        "error: The following required arguments were not provided:\n    \
          --error <MODE>\n    \
          --input <MODE>\n    \
          --output <MODE>\n\n\

tests/by-util/test_sync.rs

@@ -37,5 +37,5 @@ fn test_sync_no_existing_files() {
         .arg("--data")
         .arg("do-no-exist")
         .fails()
-        .stderr_contains("error: cannot stat");
+        .stderr_contains("cannot stat");
 }