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
1 file changed