[SPARK-26409][SQL][TESTS] SQLConf should be serializable in test sess…

…ions

## What changes were proposed in this pull request?

`SQLConf` is supposed to be serializable. However, currently it is not  serializable in `WithTestConf`. `WithTestConf` uses the method `overrideConfs` in closure, while the classes which implements it (`TestHiveSessionStateBuilder` and `TestSQLSessionStateBuilder`) are not serializable.

This PR is to use a local variable to fix it.

## How was this patch tested?

Add unit test.

Closes apache#23352 from gengliangwang/serializableSQLConf.

Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

sql/core/src/main/scala/org/apache/spark/sql/internal/BaseSessionStateBuilder.scala

@@ -309,13 +309,14 @@ private[sql] trait WithTestConf { self: BaseSessionStateBuilder =>
   def overrideConfs: Map[String, String]
 
   override protected lazy val conf: SQLConf = {
+    val overrideConfigurations = overrideConfs
     val conf = parentState.map(_.conf.clone()).getOrElse {
       new SQLConf {
         clear()
         override def clear(): Unit = {
           super.clear()
           // Make sure we start with the default test configs even after clear
-          overrideConfs.foreach { case (key, value) => setConfString(key, value) }
+          overrideConfigurations.foreach { case (key, value) => setConfString(key, value) }
         }
       }
     }

sql/core/src/test/scala/org/apache/spark/sql/SerializationSuite.scala

@@ -27,4 +27,9 @@ class SerializationSuite extends SparkFunSuite with SharedSQLContext {
     val spark = SparkSession.builder.getOrCreate()
     new JavaSerializer(new SparkConf()).newInstance().serialize(spark.sqlContext)
   }
+
+  test("[SPARK-26409] SQLConf should be serializable") {
+    val spark = SparkSession.builder.getOrCreate()
+    new JavaSerializer(new SparkConf()).newInstance().serialize(spark.sessionState.conf)
+  }
 }