Refine custom test for InstallEvent

Author: caugner

custom/tests.yaml

@@ -3293,13 +3293,17 @@ api:
       var instance = new InputEvent('input');
   InstallEvent:
     __base: |-
+      if (!('InstallEvent' in self)) {
+        return {result: false, message: 'InstallEvent is not defined'}
+      }
+
       // During the installation of the service worker, we set an "installEvent" global variable so that we may easily reuse it here:
       //   var installEvent = null;
       //   self.addEventListener('install', function (event) {
       //     installEvent = event;
       //   });
 
-      var instance = installEvent;
+      var instance = installEvent of InstallEvent ? installEvent : undefined;
   IntersectionObserver:
     IntersectionObserver:
       __additional: