fix TestAtomicUpdate to use random dir in filesystem to avoid conflict if two are running at once

git-svn-id: https://svn.apache.org/repos/asf/lucene/java/trunk@820623 13f79535-47bb-0310-9956-ffa450edef68

src/test/org/apache/lucene/index/TestAtomicUpdate.java

@@ -188,8 +188,7 @@ public class TestAtomicUpdate extends LuceneTestCase {
     directory.close();
 
     // Second in an FSDirectory:
-    String tempDir = System.getProperty("java.io.tmpdir");
-    File dirPath = new File(tempDir, "lucene.test.atomic");
+    File dirPath = _TestUtil.getTempDir("lucene.test.atomic");
     directory = FSDirectory.open(dirPath);
     runTest(directory);
     directory.close();