Add tests

tests/util.ts

@@ -4,7 +4,7 @@ import { DCCDocument, DCCXMLElement } from "../src";
 
 export function select(xpath: string, domOrDCC: JSDOM | DCCDocument): string | number | boolean | Node | Node[] {
   let dom = <JSDOM>domOrDCC;
-  if (domOrDCC.hasOwnProperty.call(null, "digitalCalibrationCertificate")) {
+  if (domOrDCC.digitalCalibrationCertificate) {
     dom = toDom(<DCCDocument>domOrDCC);
   }