[SPARK-54115][TESTS][FOLLOW-UP] Refine org.apache.spark.util.UtilsSuite

### What changes were proposed in this pull request?

### Why are the changes needed?
> Use doReturn() in those rare occasions when you cannot use when(Object)

`when` is preferred in the official doc https://javadoc.io/static/org.mockito/mockito-core/5.12.0/org/mockito/Mockito.html#when(T)

### Does this PR introduce _any_ user-facing change?
no, test-only

### How was this patch tested?
ci

### Was this patch authored or co-authored using generative AI tooling?
no

Closes #53192 from zhengruifeng/spark_54115_test_followup.

Authored-by: Ruifeng Zheng <ruifengz@apache.org>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: zhengruifeng

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -2079,7 +2079,7 @@ private[spark] object Utils
 
   val CONNECT_EXECUTE_THREAD_PREFIX = "SparkConnectExecuteThread"
 
-  private val threadInfoOrdering = Ordering.fromLessThan {
+  private[spark] val threadInfoOrdering = Ordering.fromLessThan {
     (threadTrace1: ThreadInfo, threadTrace2: ThreadInfo) => {
       def priority(ti: ThreadInfo): Int = ti.getThreadName match {
         case name if name.startsWith(TASK_THREAD_NAME_PREFIX) => 100

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -38,8 +38,7 @@ import org.apache.hadoop.fs.Path
 import org.apache.hadoop.fs.audit.CommonAuditContext.currentAuditContext
 import org.apache.hadoop.ipc.{CallerContext => HadoopCallerContext}
 import org.apache.logging.log4j.Level
-import org.mockito.Mockito.doReturn
-import org.scalatest.PrivateMethodTester
+import org.mockito.Mockito.when
 import org.scalatestplus.mockito.MockitoSugar.mock
 
 import org.apache.spark.{SparkConf, SparkException, SparkFunSuite, TaskContext}
@@ -51,7 +50,7 @@ import org.apache.spark.scheduler.SparkListener
 import org.apache.spark.util.collection.Utils.createArray
 import org.apache.spark.util.io.ChunkedByteBufferInputStream
 
-class UtilsSuite extends SparkFunSuite with ResetSystemProperties with PrivateMethodTester {
+class UtilsSuite extends SparkFunSuite with ResetSystemProperties {
 
   test("timeConversion") {
     // Test -1
@@ -1132,37 +1131,35 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with PrivateMe
 
   test("ThreadInfoOrdering") {
     val task1T = mock[ThreadInfo]
-    doReturn(11L).when(task1T).getThreadId
-    doReturn("Executor task launch worker for task 1.0 in stage 1.0 (TID 11)")
-      .when(task1T).getThreadName
-    doReturn("Executor task launch worker for task 1.0 in stage 1.0 (TID 11)")
-      .when(task1T).toString
+    when(task1T.getThreadId).thenReturn(11L)
+    when(task1T.getThreadName)
+      .thenReturn("Executor task launch worker for task 1.0 in stage 1.0 (TID 11)")
+    when(task1T.toString)
+      .thenReturn("Executor task launch worker for task 1.0 in stage 1.0 (TID 11)")
 
     val task2T = mock[ThreadInfo]
-    doReturn(12L).when(task2T).getThreadId
-    doReturn("Executor task launch worker for task 2.0 in stage 1.0 (TID 22)")
-      .when(task2T).getThreadName
-    doReturn("Executor task launch worker for task 2.0 in stage 1.0 (TID 22)")
-      .when(task2T).toString
+    when(task2T.getThreadId).thenReturn(12L)
+    when(task2T.getThreadName)
+      .thenReturn("Executor task launch worker for task 2.0 in stage 1.0 (TID 22)")
+    when(task2T.toString)
+      .thenReturn("Executor task launch worker for task 2.0 in stage 1.0 (TID 22)")
 
     val connectExecuteOp1T = mock[ThreadInfo]
-    doReturn(21L).when(connectExecuteOp1T).getThreadId
-    doReturn("SparkConnectExecuteThread_opId=16148fb4-4189-43c3-b8d4-8b3b6ddd41c7")
-      .when(connectExecuteOp1T).getThreadName
-    doReturn("SparkConnectExecuteThread_opId=16148fb4-4189-43c3-b8d4-8b3b6ddd41c7")
-      .when(connectExecuteOp1T).toString
+    when(connectExecuteOp1T.getThreadId).thenReturn(21L)
+    when(connectExecuteOp1T.getThreadName)
+      .thenReturn("SparkConnectExecuteThread_opId=16148fb4-4189-43c3-b8d4-8b3b6ddd41c7")
+    when(connectExecuteOp1T.toString)
+      .thenReturn("SparkConnectExecuteThread_opId=16148fb4-4189-43c3-b8d4-8b3b6ddd41c7")
 
     val connectExecuteOp2T = mock[ThreadInfo]
-    doReturn(22L).when(connectExecuteOp2T).getThreadId
-    doReturn("SparkConnectExecuteThread_opId=4e4d1cac-ffde-46c1-b7c2-808b726cb47e")
-      .when(connectExecuteOp2T).getThreadName
-    doReturn("SparkConnectExecuteThread_opId=4e4d1cac-ffde-46c1-b7c2-808b726cb47e")
-      .when(connectExecuteOp2T).toString
-
-    val threadInfoOrderingMethod =
-      PrivateMethod[Ordering[ThreadInfo]](Symbol("threadInfoOrdering"))
+    when(connectExecuteOp2T.getThreadId).thenReturn(22L)
+    when(connectExecuteOp2T.getThreadName)
+      .thenReturn("SparkConnectExecuteThread_opId=4e4d1cac-ffde-46c1-b7c2-808b726cb47e")
+    when(connectExecuteOp2T.toString)
+      .thenReturn("SparkConnectExecuteThread_opId=4e4d1cac-ffde-46c1-b7c2-808b726cb47e")
+
     val sorted = Seq(connectExecuteOp1T, connectExecuteOp2T, task1T, task2T)
-      .sorted(Utils.invokePrivate(threadInfoOrderingMethod()))
+      .sorted(Utils.threadInfoOrdering)
     assert(sorted === Seq(task1T, task2T, connectExecuteOp1T, connectExecuteOp2T))
   }