[SPARK-25100][TEST][FOLLOWUP] Refactor test cases in FileSuite and KryoSerializerSuite

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

Refactor test cases added by https://github.com/apache/spark/pull/26714, to improve code compactness.

### How was this patch tested?

Tested locally.

Closes #26916 from jiangxb1987/SPARK-25100.

Authored-by: Xingbo Jiang <xingbo.jiang@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/test/scala/org/apache/spark/FileSuite.scala

@@ -702,32 +702,39 @@ class FileSuite extends SparkFunSuite with LocalSparkContext {
     assert(collectRDDAndDeleteFileBeforeCompute(true).isEmpty)
   }
 
-  test("SPARK-25100: Using KryoSerializer and" +
-      "setting registrationRequired true can lead job failed") {
-    val inputFile = new File(tempDir, "/input").getAbsolutePath
-    val textFileOutputDir = new File(tempDir, "/out1").getAbsolutePath
-    val dataSetDir = new File(tempDir, "/out2").getAbsolutePath
-
-    Utils.tryWithResource(new PrintWriter(new File(inputFile))) { writer =>
-      for (i <- 1 to 100) {
+  test("SPARK-25100: Support commit tasks when Kyro registration is required") {
+    // Prepare the input file
+    val inputFilePath = new File(tempDir, "/input").getAbsolutePath
+    Utils.tryWithResource(new PrintWriter(new File(inputFilePath))) { writer =>
+      for (i <- 1 to 3) {
         writer.print(i)
         writer.write('\n')
       }
     }
 
-    val conf = new SparkConf(false).setMaster("local").
-      set("spark.kryo.registrationRequired", "true").setAppName("test")
-    conf.set("spark.serializer", classOf[KryoSerializer].getName)
+    // Start a new SparkContext
+    val conf = new SparkConf(false)
+      .setMaster("local")
+      .setAppName("test")
+      .set("spark.kryo.registrationRequired", "true")
+      .set("spark.serializer", classOf[KryoSerializer].getName)
+    sc = new SparkContext(conf)
 
+    // Prepare the input RDD
+    val pairRDD = sc.textFile(inputFilePath).map(x => (x, x))
+
+    // Test saveAsTextFile()
+    val outputFilePath1 = new File(tempDir, "/out1").getAbsolutePath
+    pairRDD.saveAsTextFile(outputFilePath1)
+    assert(sc.textFile(outputFilePath1).collect() === Array("(1,1)", "(2,2)", "(3,3)"))
+
+    // Test saveAsNewAPIHadoopDataset()
+    val outputFilePath2 = new File(tempDir, "/out2").getAbsolutePath
     val jobConf = new JobConf()
     jobConf.setOutputKeyClass(classOf[IntWritable])
     jobConf.setOutputValueClass(classOf[IntWritable])
-    jobConf.set("mapred.output.dir", dataSetDir)
-
-    sc = new SparkContext(conf)
-    val pairRDD = sc.textFile(inputFile).map(x => (x, 1))
-
-    pairRDD.saveAsTextFile(textFileOutputDir)
+    jobConf.set("mapred.output.dir", outputFilePath2)
     pairRDD.saveAsNewAPIHadoopDataset(jobConf)
+    assert(sc.textFile(outputFilePath2).collect() === Array("1\t1", "2\t2", "3\t3"))
   }
 }

core/src/test/scala/org/apache/spark/serializer/KryoSerializerSuite.scala

@@ -363,16 +363,14 @@ class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
     val conf = new SparkConf(false)
     conf.set(KRYO_REGISTRATION_REQUIRED, true)
 
+    // HadoopMapReduceCommitProtocol.commitTask() returns a TaskCommitMessage containing a complex
+    // structure.
+
     val ser = new KryoSerializer(conf).newInstance()
-    // In HadoopMapReduceCommitProtocol#commitTask
-    val addedAbsPathFiles: mutable.Map[String, String] = mutable.Map()
-    addedAbsPathFiles.put("test1", "test1")
-    addedAbsPathFiles.put("test2", "test2")
+    val addedAbsPathFiles = Map("test1" -> "test1", "test2" -> "test2")
+    val partitionPaths = Set("test3")
 
-    val partitionPaths: mutable.Set[String] = mutable.Set()
-    partitionPaths.add("test3")
-
-    val taskCommitMessage1 = new TaskCommitMessage(addedAbsPathFiles.toMap -> partitionPaths.toSet)
+    val taskCommitMessage1 = new TaskCommitMessage(addedAbsPathFiles -> partitionPaths)
     val taskCommitMessage2 = new TaskCommitMessage(Map.empty -> Set.empty)
     Seq(taskCommitMessage1, taskCommitMessage2).foreach { taskCommitMessage =>
       val obj1 = ser.deserialize[TaskCommitMessage](ser.serialize(taskCommitMessage)).obj