Add tests

tests/util.ts

+import * as xpathTs from "xpath-ts";
+import { JSDOM } from "jsdom";
+import { DCCDocument, DCCXMLElement } from "../src";
+
+export function select(xpath: string, domOrDCC: JSDOM | DCCDocument): string | number | boolean | Node | Node[] {
+  let dom = <JSDOM>domOrDCC;
+  if (domOrDCC.digitalCalibrationCertificate) {
+    dom = toDom(<DCCDocument>domOrDCC);
+  }
+
+  return xpathTs.select(xpath, dom.window.document);
+}
+
+export function toDom(dcc: DCCDocument) {
+  return new JSDOM(dcc.toXML(), { contentType: "application/xml" });
+}
+
+export function toTextArr(arr: DCCXMLElement[]) {
+  if (!arr) return [];
+  return arr.map((x) => x._text);
+}
+
+export function indexOf(arr: DCCXMLElement[], el: string) {
+  return toTextArr(arr).indexOf(el);
+}
+
+export function toTextContentArr(el: Element[]) {
+  if (!el) return [];
+  return el.map((x) => x.textContent);
+}
+
+export function undefinedIfEmpty(str) {
+  if (!str) return undefined;
+  if (str.trim() === "") return undefined;
+  return str;
+}