[SPARK-23390][SQL] Flaky Test Suite: FileBasedDataSourceSuite in Spark 2.3/hadoop 2.7

## What changes were proposed in this pull request?

This test only fails with sbt on Hadoop 2.7, I can't reproduce it locally, but here is my speculation by looking at the code:
1. FileSystem.delete doesn't delete the directory entirely, somehow we can still open the file as a 0-length empty file.(just speculation)
2. ORC intentionally allow empty files, and the reader fails during reading without closing the file stream.

This PR improves the test to make sure all files are deleted and can't be opened.

## How was this patch tested?

N/A

Author: Wenchen Fan <[email protected]>

Closes apache#20584 from cloud-fan/flaky-test.

Author: cloud-fan

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

@@ -17,6 +17,8 @@
 
 package org.apache.spark.sql
 
+import java.io.FileNotFoundException
+
 import org.apache.hadoop.fs.Path
 
 import org.apache.spark.SparkException
@@ -102,17 +104,27 @@ class FileBasedDataSourceSuite extends QueryTest with SharedSQLContext {
       def testIgnoreMissingFiles(): Unit = {
         withTempDir { dir =>
           val basePath = dir.getCanonicalPath
+
           Seq("0").toDF("a").write.format(format).save(new Path(basePath, "first").toString)
           Seq("1").toDF("a").write.format(format).save(new Path(basePath, "second").toString)
+
           val thirdPath = new Path(basePath, "third")
+          val fs = thirdPath.getFileSystem(spark.sparkContext.hadoopConfiguration)
           Seq("2").toDF("a").write.format(format).save(thirdPath.toString)
+          val files = fs.listStatus(thirdPath).filter(_.isFile).map(_.getPath)
+
           val df = spark.read.format(format).load(
             new Path(basePath, "first").toString,
             new Path(basePath, "second").toString,
             new Path(basePath, "third").toString)
 
-          val fs = thirdPath.getFileSystem(spark.sparkContext.hadoopConfiguration)
+          // Make sure all data files are deleted and can't be opened.
+          files.foreach(f => fs.delete(f, false))
           assert(fs.delete(thirdPath, true))
+          for (f <- files) {
+            intercept[FileNotFoundException](fs.open(f))
+          }
+
           checkAnswer(df, Seq(Row("0"), Row("1")))
         }
       }