[SPARK-11869][ML] Clean up TempDirectory properly in ML tests

Need to remove parent directory (```className```) rather than just tempDir (```className/random_name```)

I tested this with IDFSuite, which has 2 read/write tests, and it fixes the problem.

CC: mengxr  Can you confirm this is fine?  I believe it is since the same ```random_name``` is used for all tests in a suite; we basically have an extra unneeded level of nesting.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #9851 from jkbradley/tempdir-cleanup.

mllib/src/test/scala/org/apache/spark/ml/util/TempDirectory.scala

@@ -35,7 +35,7 @@ trait TempDirectory extends BeforeAndAfterAll { self: Suite =>
 
   override def beforeAll(): Unit = {
     super.beforeAll()
-    _tempDir = Utils.createTempDir(this.getClass.getName)
+    _tempDir = Utils.createTempDir(namePrefix = this.getClass.getName)
   }
 
   override def afterAll(): Unit = {