[Performance #285] fix faulty test and ensure that the task is null a…

…fter transferring out of throttled future

src/main/java/cz/cvut/kbss/termit/util/throttle/ThrottledFuture.java

@@ -148,6 +148,7 @@ protected ThrottledFuture<T> update(Supplier<T> task, @NonNull List<Consumer<T>>
     /**
      * Returns future with the task from the specified {@code throttledFuture}.
      * If possible, transfers the task from this object to the specified {@code throttledFuture}.
+     * If the task was successfully transferred, this future is canceled.
      *
      * @param target the future to update
      * @return target when current future is already being executed, was canceled or completed.

src/test/java/cz/cvut/kbss/termit/util/throttle/ThrottledFutureTest.java

@@ -281,8 +281,9 @@ void transferUpdatesSecondFutureWithTask() {
         verify(secondFuture).update(eq(firstTask), anyList());
 
         // now verifies that the task in the first future is null
-        firstFuture.transfer(secondFuture);
-        verify(secondFuture).update(isNull(), anyList());
+        Object task = ReflectionTestUtils.getField(firstFuture, "task");
+        assertNull(task);
+        assertTrue(firstFuture.isCancelled());
     }
 
     @Test