[SPARK-21790][TESTS][FOLLOW-UP] Add filter pushdown verification back.

## What changes were proposed in this pull request?

The previous PR(https://github.com/apache/spark/pull/19000) removed filter pushdown verification, This PR add them back.

## How was this patch tested?
manual tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #19002 from wangyum/SPARK-21790-follow-up.

external/docker-integration-tests/src/test/scala/org/apache/spark/sql/jdbc/OracleIntegrationSuite.scala

@@ -22,6 +22,7 @@ import java.util.Properties
 import java.math.BigDecimal
 
 import org.apache.spark.sql.Row
+import org.apache.spark.sql.execution.{WholeStageCodegenExec, RowDataSourceScanExec}
 import org.apache.spark.sql.test.SharedSQLContext
 import org.apache.spark.sql.types._
 import org.apache.spark.tags.DockerTest
@@ -255,6 +256,18 @@ class OracleIntegrationSuite extends DockerJDBCIntegrationSuite with SharedSQLCo
     val df = dfRead.filter(dfRead.col("date_type").lt(dt))
       .filter(dfRead.col("timestamp_type").lt(ts))
 
+    val parentPlan = df.queryExecution.executedPlan
+    assert(parentPlan.isInstanceOf[WholeStageCodegenExec])
+    val node = parentPlan.asInstanceOf[WholeStageCodegenExec]
+    val metadata = node.child.asInstanceOf[RowDataSourceScanExec].metadata
+    // The "PushedFilters" part should exist in Dataframe's
+    // physical plan and the existence of right literals in
+    // "PushedFilters" is used to prove that the predicates
+    // pushing down have been effective.
+    assert(metadata.get("PushedFilters").isDefined)
+    assert(metadata("PushedFilters").contains(dt.toString))
+    assert(metadata("PushedFilters").contains(ts.toString))
+
     val row = df.collect()(0)
     assert(row.getDate(0).equals(dateVal))
     assert(row.getTimestamp(1).equals(timestampVal))