Fix failure in ScalingThreadPoolTests after addition of merge scheduler (#125245)

Author: mark-vieira

server/src/test/java/org/elasticsearch/threadpool/ScalingThreadPoolTests.java

@@ -119,6 +119,7 @@ private int expectedSize(final String threadPoolName, final int numberOfProcesso
         sizes.put(ThreadPool.Names.SNAPSHOT_META, n -> Math.min(n * 3, 50));
         sizes.put(ThreadPool.Names.FETCH_SHARD_STARTED, ThreadPool::twiceAllocatedProcessors);
         sizes.put(ThreadPool.Names.FETCH_SHARD_STORE, ThreadPool::twiceAllocatedProcessors);
+        sizes.put(ThreadPool.Names.MERGE, Function.identity());
         return sizes.get(threadPoolName).apply(numberOfProcessors);
     }