SearchServiceTests.testIsParallelCollectionSupportedForResults fails intermittently

```
12:37:10 org.elasticsearch.search.SearchServiceTests > testIsParallelCollectionSupportedForResults FAILED
12:37:10     java.lang.AssertionError
12:37:10         at __randomizedtesting.SeedInfo.seed([4140C168DE8FB0BB:771A78D5E4B36180]:0)
12:37:10         at org.junit.Assert.fail(Assert.java:86)
12:37:10         at org.junit.Assert.assertTrue(Assert.java:41)
12:37:10         at org.junit.Assert.assertFalse(Assert.java:64)
12:37:10         at org.junit.Assert.assertFalse(Assert.java:74)
12:37:10         at org.elasticsearch.search.SearchServiceTests.testIsParallelCollectionSupportedForResults(SearchServiceTests.java:2096)
```

```
./gradlew ':server:test' --tests "org.elasticsearch.search.SearchServiceTests.testIsParallelCollectionSupportedForResults" -Dtests.seed=4140C168DE8FB0BB -Dtests.locale=es-SV -Dtests.timezone=Africa/Banjul -Druntime.java=20
```
server/src/test/java/org/elasticsearch/search/SearchServiceTests.java
.get();
            assertTrue(response.isAcknowledged());
            {
                assertEquals(executorPoolSize, service.determineMaximumNumberOfSlices(threadPoolExecutor, request, ResultsType.DFS));
                assertEquals(1, service.determineMaximumNumberOfSlices(null, request, ResultsType.DFS));
                assertEquals(executorPoolSize, service.determineMaximumNumberOfSlices(threadPoolExecutor, request, ResultsType.QUERY));
                assertEquals(1, service.determineMaximumNumberOfSlices(null, request, ResultsType.QUERY));
                assertEquals(1, service.determineMaximumNumberOfSlices(notThreadPoolExecutor, request, ResultsType.DFS));
            }
        } finally {
            
            client().admin()
                .cluster()
                .prepareUpdateSettings()
                .setPersistentSettings(Settings.builder().putNull(QUERY_PHASE_PARALLEL_COLLECTION_ENABLED.getKey()).build())
                .get();
            {
                assertEquals(executorPoolSize, service.determineMaximumNumberOfSlices(threadPoolExecutor, request, ResultsType.DFS));
                assertEquals(1, service.determineMaximumNumberOfSlices(threadPoolExecutor, request, ResultsType.QUERY));
            }
        }
    }
    /**
     * Verify that a single slice is created for requests that don't support parallel collection, while computation
     * is still offloaded to the worker threads. Also ensure multiple slices are created for requests that do support
     * parallel collection.
     */
    public void testSlicingBehaviourForParallelCollection() throws Exception {
        IndexService indexService = createIndex("index", Settings.EMPTY);
        ThreadPoolExecutor executor = (ThreadPoolExecutor) indexService.getThreadPool().executor(ThreadPool.Names.SEARCH_WORKER);