MB-21428: Make sure all tasks in fake executor pool finish execution in teardown

It is not sufficient to just run all tasks in future and ready queues while
shutdown, we also need to make sure they finish before delete
EventuallyPersistentEngine instance.

Change-Id: I658a2176b6f4a99beacbc8dd2e010b93c4480eaa
Reviewed-on: http://review.couchbase.org/68968
Tested-by: buildbot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>
Reviewed-by: Jim Walker <[email protected]>

Author: manudhundi

tests/module_tests/evp_store_single_threaded_test.cc

@@ -102,6 +102,8 @@ class SingleThreadedEPStoreTest : public EventuallyPersistentStoreTest {
                 }
             };
             runTasks(*task_executor->getLpTaskQ()[t]);
+            task_executor->stopTaskGroup(engine->getTaskable().getGID(), t,
+                                         engine->getEpStats().forceShutdown);
         }
     }