Fix PrioritizedThrottledTaskRunnerTests

These tests try and execute `maxThreads` concurrent tasks to ensure that
the rest of the executor's queue has been processed, but due to elastic#93443
(and the executor's zero timeout) this sometimes doesn't work. This
commit fixes the problem by making every thread a core thread so that
they do not time out.

Closes elastic#92910

server/src/test/java/org/elasticsearch/common/util/concurrent/PrioritizedThrottledTaskRunnerTests.java

@@ -39,7 +39,7 @@ public class PrioritizedThrottledTaskRunnerTests extends ESTestCase {
     public void setUp() throws Exception {
         super.setUp();
         maxThreads = between(1, 10);
-        executor = EsExecutors.newScaling("test", 1, maxThreads, 0, TimeUnit.MILLISECONDS, false, threadFactory, threadContext);
+        executor = EsExecutors.newScaling("test", maxThreads, maxThreads, 0, TimeUnit.NANOSECONDS, false, threadFactory, threadContext);
     }
 
     @Override