M6: fill in the list selection fields in the engine's XFA test helper

Main's new engine test builds a pdfcraft_forms::Field literal; give it
the selected and single_value_is_array fields this branch adds.

crates/engine/src/xfa.rs

@@ -1051,6 +1051,8 @@ mod tests {
             flags: 0,
             max_len: None,
             options: Vec::new(),
+            selected: Vec::new(),
+            single_value_is_array: false,
             da: String::new(),
             quadding: 0,
             tooltip: None,