add util to wait for tab loading

e2e/ambassador/src/background/tabs.js

@@ -0,0 +1,18 @@
+const create = (props = {}) => {
+  return new Promise((resolve) => {
+    browser.tabs.create(props).then((createdTab) => {
+      let callback = (tabId, changeInfo, tab) => {
+        if (tab.url !== 'about:blank' && tabId === createdTab.id &&
+            changeInfo.status === 'complete') {
+          browser.tabs.onUpdated.removeListener(callback);
+          resolve(tab);
+        }
+      };
+      browser.tabs.onUpdated.addListener(callback);
+    });
+  });
+};
+
+export {
+  create,
+};