Update core/src/test/java/com/datastax/dse/driver/internal/core/cql/reactive/TestSubscriber.java

Co-authored-by: Alexandre Dutra <[email protected]>

Author: dssysolyatin

core/src/test/java/com/datastax/dse/driver/internal/core/cql/reactive/TestSubscriber.java

@@ -31,7 +31,7 @@ public class TestSubscriber<T> implements Subscriber<T> {
 
   private final List<T> elements = new ArrayList<>();
   private final CountDownLatch latch = new CountDownLatch(1);
-  private long demand;
+  private final long demand;
   private boolean isCompleted;
   private Subscription subscription;
   private Throwable error;