PR in response to a set of tests deemed flaky by TeamCity CI. Example here: http://petabridge-ci.cloudapp.net/viewLog.html?buildId=21833&tab=buildResultsDiv&buildTypeId=AkkaNet_AkkaPersistenceImplementations_AkkaPersistenceSqlServer_PrBuilds#testNameId-488764090331667693

Test failures were due to the DeleteCommand being sent to the TestActor not completing the Journal cleanup before the assertions were being made against the cleaned-up Journal.

Failed: Expected a message of type Akka.Streams.TestKit.TestSubscriber+OnComplete, 
but received {TestSubscriber.OnNext(g1-1)} 
(type Akka.Streams.TestKit.TestSubscriber+OnNext`1[System.Object]) instead  
from [akka://test/user/StreamSupervisor-16/Flow-0-0-select#2080382836]

Revised TestActor to send acknowledgement to TestKit actor only after it has received the DeleteMessagesSuccess message back from the Journal. Also added AwaitAssert to wait for the ExpectMsg("-deleted") message or fail if not received within default timespan.