[SPARK-21516][SQL][TEST] Overriding afterEach() in DatasetCacheSuite must call super.afterEach()

## What changes were proposed in this pull request?

This PR ensures to call `super.afterEach()` in overriding `afterEach()` method in `DatasetCacheSuite`. When we override `afterEach()` method in Testsuite, we have to call `super.afterEach()`.

This is a follow-up of #18719 and SPARK-21512.

## How was this patch tested?

Used the existing test suite

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #18721 from kiszk/SPARK-21516.

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

@@ -25,11 +25,6 @@ import org.apache.spark.storage.StorageLevel
 class DatasetCacheSuite extends QueryTest with SharedSQLContext {
   import testImplicits._
 
-  // Clear all persistent datasets after each test
-  override def afterEach(): Unit = {
-    spark.sharedState.cacheManager.clearCache()
-  }
-
   test("get storage level") {
     val ds1 = Seq("1", "2").toDS().as("a")
     val ds2 = Seq(2, 3).toDS().as("b")

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

@@ -87,6 +87,8 @@ trait SharedSQLContext extends SQLTestUtils with BeforeAndAfterEach with Eventua
 
   protected override def afterEach(): Unit = {
     super.afterEach()
+    // Clear all persistent datasets after each test
+    spark.sharedState.cacheManager.clearCache()
     // files can be closed from other threads, so wait a bit
     // normally this doesn't take more than 1s
     eventually(timeout(10.seconds)) {