tests: update test_df.rs to use --output argument

tests/by-util/test_df.rs

@@ -40,21 +40,14 @@ fn test_df_output() {
 /// Test that the order of rows in the table does not change across executions.
 #[test]
 fn test_order_same() {
-    // TODO When #3057 is resolved, we should just use
-    //
-    //     new_ucmd!().arg("--output=source").succeeds().stdout_move_str();
-    //
-    // instead of parsing the entire `df` table as a string.
-    let output1 = new_ucmd!().succeeds().stdout_move_str();
-    let output2 = new_ucmd!().succeeds().stdout_move_str();
-    let output1: Vec<String> = output1
-        .lines()
-        .map(|l| String::from(l.split_once(' ').unwrap().0))
-        .collect();
-    let output2: Vec<String> = output2
-        .lines()
-        .map(|l| String::from(l.split_once(' ').unwrap().0))
-        .collect();
+    let output1 = new_ucmd!()
+        .arg("--output=source")
+        .succeeds()
+        .stdout_move_str();
+    let output2 = new_ucmd!()
+        .arg("--output=source")
+        .succeeds()
+        .stdout_move_str();
     assert_eq!(output1, output2);
 }