Fix intermittent testAddShardReplicaWithAddServerReplyFailure failure

Fix intermittent testAddShardReplicaWithAddServerReplyFailure failure

ShardManagerTest#testAddShardReplicaWithAddServerReplyFailure failed:

java.lang.AssertionError: assertion failed: timeout (3 seconds) during expectMsgClass waiting for class org.opendaylight.controller.cluster.raft.messages.AddServer
20:14:24 	at scala.Predef$.assert(Predef.scala:170)
20:14:24 	at akka.testkit.TestKitBase$class.expectMsgClass_internal(TestKit.scala:472)
20:14:24 	at akka.testkit.TestKitBase$class.expectMsgClass(TestKit.scala:459)
20:14:24 	at akka.testkit.TestKit.expectMsgClass(TestKit.scala:814)
20:14:24 	at akka.testkit.JavaTestKit.expectMsgClass(JavaTestKit.java:415)
20:14:24 	at org.opendaylight.controller.cluster.datastore.shardmanager.ShardManagerTest$33.<init>(ShardManagerTest.java:1637)

The log shows:

08:14:06,302 PM [main] [INFO] ShardManagerTest - testAddShardReplicaWithAddServerReplyFailure starting
08:14:06,325 PM [main] [INFO] ShardManager - Starting ShardManager shard-manager-config22
08:14:06,329 PM [test-akka.actor.default-dispatcher-7] [INFO] ShardManager - Recovery complete : shard-manager-config22
08:14:09,339 PM [main] [INFO] TestActorFactory - Killing actor TestActor[akka://test/user/member-1-shard-astronauts-config]
08:14:09,340 PM [main] [INFO] TestActorFactory - Killing actor TestActor[akka://test/user/shardmanager-config22]
08:14:09,340 PM [main] [DEBUG] ShardManager - Got updated SchemaContext: # of modules 1
08:14:09,340 PM [main] [DEBUG] ShardManager - shard-manager-config22: onAddShardReplica: AddShardReplica[ShardName=astronauts]
08:14:09,340 PM [main] [INFO] ShardManager - Stopping ShardManager shard-manager-config22

So the ShardManager got the onAddShardReplica message but after the test timed out
after 3 seconds. The problem is that the test is using the default dispatcher for
TestActor which is the calling thread dispatcher which is problematic for persistent
actors. Either not use TestActor where we don't need access to the underlying actor
instance or use the system default dispatcher, which is async.

Change-Id: Ib6521c345bd0db9502d0078928f8d0e5dcd7f747
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>