Bug 1745929 - followup - Properly clean up in new test. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D134013

--HG--
extra : amend_source : 3081c71b5eb463f78bdfd4e9b2b8499243d4db88

chat/protocols/matrix/test/test_matrixRoom.js

@@ -551,6 +551,7 @@ add_task(function test_createMessage() {
   ok(message.containsNick);
   equal(message.time, Math.floor(time / 1000));
   equal(message.iconURL, "https://example.com/avatar");
+  roomStub.forget();
 });
 
 function waitForNotification(target, expectedTopic) {