[SC-4372] Set correct taskcontext in async-io thread

## What changes were proposed in this pull request?

This patch fixes a broken test in `ParquetFilterSuite`.

## How was this patch tested?

`ParquetFilterSuite`

Author: Sameer Agarwal <sameerag@cs.berkeley.edu>

Closes apache#68 from sameeragarwal/async-io.

Author: sameeragarwal

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileScanRDD.scala

@@ -178,8 +178,10 @@ class FileScanRDD(
       }
 
       def prepareNextFile(): Future[NextFile] = {
+        val taskContext = TaskContext.get()
         if (files.hasNext) {
           Future {
+            TaskContext.setTaskContext(taskContext)
             val nextFile = files.next()
             val nextFileIter =
               try {

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFilterSuite.scala

@@ -563,7 +563,7 @@ class ParquetFilterSuite extends QueryTest with ParquetTest with SharedSQLContex
     }
   }
 
-  test("Fiters should be pushed down for vectorized Parquet reader at row group level") {
+  test("Filters should be pushed down for vectorized Parquet reader at row group level") {
     import testImplicits._
 
     withSQLConf(SQLConf.PARQUET_VECTORIZED_READER_ENABLED.key -> "true",