test: add format filesize gigabytes test

src/builtins/ls.rs

@@ -122,4 +122,9 @@ mod tests {
     fn test_format_filesize_megabytes() {
         assert_eq!(format_filesize(69420420), "69MB");
     }
+
+    #[test]
+    fn test_format_filesize_gigabytes() {
+        assert_eq!(format_filesize(69420420420), "69GB");
+    }
 }