Adds a test for sync.

It is probably too hard to verify that the sync is actually performed,
so we just check that we have a test using the code path, pro forma.

Signed-off-by: anastygnome <noreplygitemail@protonmail.com>

tests/by-util/test_df.rs

@@ -28,6 +28,11 @@ fn test_df_compatible_si() {
     new_ucmd!().arg("-aH").succeeds();
 }
 
+#[test]
+fn test_df_compatible_sync() {
+    new_ucmd!().arg("--sync").succeeds();
+}
+
 #[test]
 fn test_df_arguments_override_themselves() {
     new_ucmd!().args(&["--help", "--help"]).succeeds();