[SPARK-1511] use Files.move instead of renameTo in TestUtils.scala

JIRA issue:[SPARK-1511](https://issues.apache.org/jira/browse/SPARK-1511)

TestUtils.createCompiledClass method use renameTo() to move files which fails when the src and dest files are in different disks or partitions. This pr uses Files.move() instead. The move method will try to use renameTo() and then fall back to copy() and delete(). I think this should handle this issue.

I didn't found a test suite for this file, so I add file existence detection after file moving.

Author: Ye Xianjin <advancedxy@gmail.com>

Closes #427 from advancedxy/SPARK-1511 and squashes the following commits:

a2b97c7 [Ye Xianjin] Based on @srowen's comment, assert file existence.
6f95550 [Ye Xianjin] use Files.move instead of renameTo to handle the src and dest files are in different disks or partitions.

core/src/main/scala/org/apache/spark/TestUtils.scala

@@ -100,9 +100,14 @@ private[spark] object TestUtils {
 
     val fileName = className + ".class"
     val result = new File(fileName)
-    if (!result.exists()) throw new Exception("Compiled file not found: " + fileName)
+    assert(result.exists(), "Compiled file not found: " + result.getAbsolutePath())
     val out = new File(destDir, fileName)
-    result.renameTo(out)
+
+    // renameTo cannot handle in and out files in different filesystems
+    // use google's Files.move instead
+    Files.move(result, out)
+
+    assert(out.exists(), "Destination file not moved: " + out.getAbsolutePath())
     out
   }
 }