Use java.util.concurrent.ScheduledThreadPoolExecutor for tests

The stable-2.14 version of the plugin used
`com.google.gerrit.server.git.WorkQueue.Executor` to mock an executor
during tests. This executor implementation does not exist anymore in
Gerrit 2.15. Thus, `java.util.concurrent.ScheduledThreadPoolExecutor`
is now used.

Change-Id: I03579b626e6592effde4aac70730283333384c50

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

diff --git a/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheCleanerTest.java b/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheCleanerTest.java
index 5761bcc..de32dcd 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheCleanerTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheCleanerTest.java
@@ -25,10 +25,10 @@
 import static org.mockito.Mockito.when;
 
 import com.google.gerrit.server.git.WorkQueue;
-import com.google.gerrit.server.git.WorkQueue.Executor;
 import com.google.inject.Provider;
 import com.googlesource.gerrit.plugins.websession.flatfile.FlatFileWebSessionCacheCleaner.CleanupTask;
 import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 import org.junit.Before;
 import org.junit.Test;
@@ -42,7 +42,7 @@
   private static long CLEANUP_INTERVAL = 5000;
   private static String SOME_PLUGIN_NAME = "somePluginName";
 
-  @Mock private Executor executorMock;
+  @Mock private ScheduledThreadPoolExecutor executorMock;
   @Mock private ScheduledFuture<?> scheduledFutureMock;
   @Mock private WorkQueue workQueueMock;
   @Mock private Provider<CleanupTask> cleanupTaskProviderMock;