Bug 1497929 - Await the underlying promise r=chutten

Previously, the test never waited for the notification to actually fire.

Differential Revision: https://phabricator.services.mozilla.com/D8249

--HG--
extra : moz-landing-system : lando

toolkit/components/telemetry/tests/unit/test_TelemetryController_idle.js

@@ -49,7 +49,7 @@ add_task(async function testSendPendingOnIdleDaily() {
 
   // Check that we are correctly receiving the gather-telemetry notification.
   TelemetrySession.observe(null, "idle-daily", null);
-  await gatherPromise;
+  await gatherPromise.promise;
   Assert.ok(true, "Received gather-telemetry notification.");
 
   Services.obs.removeObserver(gatherPromise.resolve, "gather-telemetry");