[SPARK-23826][TEST] TestHiveSparkSession should set default session

## What changes were proposed in this pull request?
In TestHive, the base spark session does this in getOrCreate(), we emulate that behavior for tests.

## How was this patch tested?
N/A

Author: gatorsmile <[email protected]>

Closes apache#20969 from gatorsmile/setDefault.

Author: gatorsmile

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

@@ -159,10 +159,6 @@ 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,