alert_tab: clear test result when testing rules

closes #9539

public/app/features/alerting/alert_tab_ctrl.ts

@@ -383,6 +383,7 @@ export class AlertTabCtrl {
 
   test() {
     this.testing = true;
+    this.testResult = false;
 
     var payload = {
       dashboard: this.dashboardSrv.getCurrent().getSaveModelClone(),