[SPARK-20609][CORE] Run the SortShuffleSuite unit tests have residual spark_* system directory

## What changes were proposed in this pull request?
This PR solution to run the SortShuffleSuite unit tests have residual spark_* system directory
For example:
OS:Windows 7
After the running SortShuffleSuite unit tests,
the system of TMP directory have '..\AppData\Local\Temp\spark-f64121f9-11b4-4ffd-a4f0-cfca66643503' not deleted

## How was this patch tested?
Run SortShuffleSuite unit test.

Author: caoxuewen <cao.xuewen@zte.com.cn>

Closes apache#17869 from heary-cao/SortShuffleSuite.

Author: heary-cao

core/src/test/scala/org/apache/spark/SortShuffleSuite.scala

@@ -38,6 +38,10 @@ class SortShuffleSuite extends ShuffleSuite with BeforeAndAfterAll {
 
   override def beforeAll() {
     super.beforeAll()
+    // Once 'spark.local.dir' is set, it is cached. Unless this is manually cleared
+    // before/after a test, it could return the same directory even if this property
+    // is configured.
+    Utils.clearLocalRootDirs()
     conf.set("spark.shuffle.manager", "sort")
   }
 
@@ -50,6 +54,7 @@ class SortShuffleSuite extends ShuffleSuite with BeforeAndAfterAll {
   override def afterEach(): Unit = {
     try {
       Utils.deleteRecursively(tempDir)
+      Utils.clearLocalRootDirs()
     } finally {
       super.afterEach()
     }