Make mutable static field volatile, in case used from multiple threads

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/io/trunk@919238 13f79535-47bb-0310-9956-ffa450edef68

Author: sebbASF

src/test/org/apache/commons/io/testtools/FileBasedTestCase.java

@@ -43,7 +43,7 @@
  */
 public abstract class FileBasedTestCase extends TestCase {
 
-    private static File testDir;
+    private static volatile File testDir;
 
     public FileBasedTestCase(String name) {
         super(name);