tests(df): Fix test_total to avoid whitespace parsing issues

Improve test_total robustness by using --output=size,used,avail to get
numeric-only columns, avoiding fragile whitespace parsing of Filesystem
and Mounted on columns which can contain spaces in mount point names.

This makes the test more reliable across different filesystem configurations
and mount point naming conventions.

tests/by-util/test_df.rs

@@ -395,34 +395,36 @@ fn test_total() {
     //     ...
     //     /dev/loop14               63488    63488         0 100% /snap/core20/1361
     //     total                 258775268 98099712 148220200  40% -
-    let output = new_ucmd!().arg("--total").succeeds().stdout_str_lossy();
+    // Use explicit numeric-only columns to avoid whitespace issues in Filesystem or Mounted on columns
+    let output = new_ucmd!()
+        .arg("--output=size,used,avail")
+        .arg("--total")
+        .succeeds()
+        .stdout_str_lossy();
 
     // Skip the header line.
     let lines: Vec<&str> = output.lines().skip(1).collect();
 
-    // Parse the values from the last row.
+    // Parse the values from the last row (report totals)
     let last_line = lines.last().unwrap();
     let mut iter = last_line.split_whitespace();
-    assert_eq!(iter.next().unwrap(), "total");
-    let reported_total_size = iter.next().unwrap().parse().unwrap();
-    let reported_total_used = iter.next().unwrap().parse().unwrap();
-    let reported_total_avail = iter.next().unwrap().parse().unwrap();
+    let reported_total_size: u64 = iter.next().unwrap().parse().unwrap();
+    let reported_total_used: u64 = iter.next().unwrap().parse().unwrap();
+    let reported_total_avail: u64 = iter.next().unwrap().parse().unwrap();
 
     // Loop over each row except the last, computing the sum of each column.
-    let mut computed_total_size = 0;
-    let mut computed_total_used = 0;
-    let mut computed_total_avail = 0;
+    let mut computed_total_size: u64 = 0;
+    let mut computed_total_used: u64 = 0;
+    let mut computed_total_avail: u64 = 0;
     let n = lines.len();
     for line in &lines[..n - 1] {
         let mut iter = line.split_whitespace();
-        iter.next().unwrap();
         computed_total_size += iter.next().unwrap().parse::<u64>().unwrap();
         computed_total_used += iter.next().unwrap().parse::<u64>().unwrap();
         computed_total_avail += iter.next().unwrap().parse::<u64>().unwrap();
     }
 
-    // Check that the sum of each column matches the reported value in
-    // the last row.
+    // Check that the sum of each column matches the reported value in the last row.
     assert_eq!(computed_total_size, reported_total_size);
     assert_eq!(computed_total_used, reported_total_used);
     assert_eq!(computed_total_avail, reported_total_avail);