Change TestKitBase.InitializeTest to virtual so it can be deferred (#…

…6093) (#6095)

(cherry picked from commit 63639b9)

src/core/Akka.TestKit/TestKitBase.cs

@@ -127,7 +127,7 @@ protected TestKitBase(ITestKitAssertions assertions, ActorSystem system, ActorSy
         /// <param name="config">The configuration that <paramref name="system"/> will use if it's null.</param>
         /// <param name="actorSystemName">The name that <paramref name="system"/> will use if it's null.</param>
         /// <param name="testActorName">The name of the test actor. Can be null.</param>
-        protected void InitializeTest(ActorSystem system, ActorSystemSetup config, string actorSystemName, string testActorName)
+        protected virtual void InitializeTest(ActorSystem system, ActorSystemSetup config, string actorSystemName, string testActorName)
         {
             _testState = new TestState();