Bug 1390441 - Fix usePingSenderOnShutdown test via reseting TelemetrySend in tests. r=gfritzsche

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

@@ -304,6 +304,7 @@ add_task(async function test_usePingSenderOnShutdown() {
   }
 
   TelemetryHealthPing.testReset();
+  await TelemetrySend.reset();
   PingServer.clearRequests();
 
   // This first failure should immediately trigger a ping.