[SPARK-19669][HOTFIX][SQL] sessionState access privileges compiled failed in TestSQLContext

## What changes were proposed in this pull request?

In [SPARK-19669](https://github.com/apache/spark/commit/0733a54a4517b82291efed9ac7f7407d9044593c) change the sessionState access privileges from private to public, this lead to the compile failed in TestSQLContext

this pr is a hotfix for this.

## How was this patch tested?
N/A

Author: windpiger <songjun@outlook.com>

Closes #17008 from windpiger/hotfixcompile.

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

@@ -35,7 +35,7 @@ private[sql] class TestSparkSession(sc: SparkContext) extends SparkSession(sc) {
   }
 
   @transient
-  protected[sql] override lazy val sessionState: SessionState = new SessionState(self) {
+  override lazy val sessionState: SessionState = new SessionState(self) {
     override lazy val conf: SQLConf = {
       new SQLConf {
         clear()