[SPARK-23808][SQL] Set default Spark session in test-only spark sessions.

## What changes were proposed in this pull request?

Set default Spark session in the TestSparkSession and TestHiveSparkSession constructors.

## How was this patch tested?

new unit tests

Author: Jose Torres <[email protected]>

Closes apache#20926 from jose-torres/test3.

Author: jose-torres

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

@@ -34,6 +34,8 @@ private[spark] class TestSparkSession(sc: SparkContext) extends SparkSession(sc)
     this(new SparkConf)
   }
 
+  SparkSession.setDefaultSession(this)
+
   @transient
   override lazy val sessionState: SessionState = {
     new TestSQLSessionStateBuilder(this, None).build()

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

@@ -0,0 +1,29 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.test
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.SparkSession
+
+class TestSparkSessionSuite extends SparkFunSuite {
+  test("default session is set in constructor") {
+    val session = new TestSparkSession()
+    assert(SparkSession.getDefaultSession.contains(session))
+    session.stop()
+  }
+}

sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala

@@ -159,6 +159,10 @@ private[hive] class TestHiveSparkSession(
     private val loadTestTables: Boolean)
   extends SparkSession(sc) with Logging { self =>
 
+  // TODO(SPARK-23826): TestHiveSparkSession should set default session the same way as
+  // TestSparkSession, but doing this the same way breaks many tests in the package. We need
+  // to investigate and find a different strategy.
+
   def this(sc: SparkContext, loadTestTables: Boolean) {
     this(
       sc,