[SPARK-23671][CORE] Fix condition to enable the SHS thread pool.

Author: Marcelo Vanzin <[email protected]>

Closes #20814 from vanzin/SPARK-23671.

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/deploy/history/FsHistoryProvider.scala

@@ -173,7 +173,7 @@ private[history] class FsHistoryProvider(conf: SparkConf, clock: Clock)
    * Fixed size thread pool to fetch and parse log files.
    */
   private val replayExecutor: ExecutorService = {
-    if (Utils.isTesting) {
+    if (!Utils.isTesting) {
       ThreadUtils.newDaemonFixedThreadPool(NUM_PROCESSING_THREADS, "log-replay-executor")
     } else {
       MoreExecutors.sameThreadExecutor()