Add and fixup invite link related tests

src/test/common.js

@@ -208,5 +208,9 @@ function clearMailQueue() {
 async function checkMails(number) {
     await delay(1000);
     expect(mailer._mailQueue.length).to.equal(number);
+    const emails = mailer._mailQueue;
     clearMailQueue();
+
+    // return for further investigation
+    return emails;
 }