Use junit to provide a temporary directory for testing

So far the java.nio.Files was used to create a cache directory during
tests. Junit also provides a functionality to create temporary
directories for tests.

This change implements the usage of a junit rule to create a temporary
directory. This also increases the similarity to the HA-plugin.

Change-Id: Id184b12ab1abf6d0ca355c41f3313ff583c916a3

src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java b/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java
index 1bafca8..b075bff 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java
@@ -37,14 +37,17 @@
 import java.util.Map;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
-import org.junit.After;
 import org.junit.Before;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
 
 public class FlatFileWebSessionCacheTest {
 
   private static final int DEFAULT_KEYS_SIZE = 10;
 
+  @Rule public TemporaryFolder tempFolder = new TemporaryFolder();
+
   private FlatFileWebSessionCache flatFileWebSessionCache;
   private Path dir;
   private String key;
@@ -52,21 +55,12 @@
 
   @Before
   public void createFlatFileWebSessionCache() throws Exception {
-    dir = Files.createTempDirectory("websessions");
+    dir = tempFolder.newFolder("websessions").toPath();
     key = "aOc2prqlZRpSO3LpauGO5efCLs1L9r9KkG";
     existingKey = "aSceprtBc02YaMY573T5jfW64ZudJfPbDq";
     flatFileWebSessionCache = new FlatFileWebSessionCache(dir);
   }
 
-  @After
-  public void tearDown() throws Exception {
-    if (isDirEmpty(dir)) {
-      Files.deleteIfExists(dir);
-    } else {
-      emptyAndDelete(dir);
-    }
-  }
-
   @Test
   public void asMapTest() throws Exception {
     Files.createFile(dir.resolve(key));