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