samples: Flaky test fix (#1490)

* Rework the tests to remove flakiness (hopefully)

* Update IDs since old test deleted my knowledge base

* Update GCS links to fix permission denied errors

* Use declared var

Author: nnegrey

samples/snippets/src/test/java/com/example/video/DetectIT.java

@@ -84,7 +84,8 @@ public void testTrackObjects() throws Exception {
 
   @Test
   public void testTrackObjectsGcs() throws Exception {
-    VideoAnnotationResults result = TrackObjects.trackObjectsGcs("gs://demomaker/cat.mp4");
+    VideoAnnotationResults result = TrackObjects.trackObjectsGcs(
+            "gs://cloud-samples-data/video/cat.mp4");
 
     boolean textExists = false;
     for (ObjectTrackingAnnotation objectTrackingAnnotation : result.getObjectAnnotationsList()) {