[Akka.TestKit] TestKitBase should take arbitrary ActorSystem without …

…throwing (#6092)

(cherry picked from commit 015de37)

src/core/Akka.TestKit.Tests/TestKitSpec.cs

@@ -0,0 +1,59 @@
+// -----------------------------------------------------------------------
+//  <copyright file="TestKitSpec.cs" company="Akka.NET Project">
+//      Copyright (C) 2009-2022 Lightbend Inc. <http://www.lightbend.com>
+//      Copyright (C) 2013-2022 .NET Foundation <https://github.com/akkadotnet/akka.net>
+//  </copyright>
+// -----------------------------------------------------------------------
+
+using Akka.Actor;
+using Akka.Actor.Dsl;
+using Akka.Configuration;
+using Xunit;
+using Xunit.Abstractions;
+using FluentAssertions;
+using static FluentAssertions.FluentActions;
+
+namespace Akka.TestKit.Tests
+{
+    public class TestKitSpec
+    {
+        private readonly ITestOutputHelper _output;
+
+        public TestKitSpec(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        [Fact(DisplayName = "TestKit should accept arbitrary ActorSystem")]
+        public void TestKitBaseTest()
+        {
+            using (var sys = ActorSystem.Create(nameof(TestKitSpec)))
+            {
+                var testkit = new TestKit.Xunit2.TestKit(sys, _output);
+                var echoActor = testkit.Sys.ActorOf(c => c.ReceiveAny((m, ctx) => testkit.TestActor.Tell(m))); 
+                Invoking(() =>
+                {
+                    echoActor.Tell("message");
+                    var message = testkit.ExpectMsg<string>();
+                    message.Should().Be("message");
+                }).Should().NotThrow<ConfigurationException>();
+            }
+        }
+
+        [Fact(DisplayName = "TestKit should accept ActorSystem with TestKit.DefaultConfig")]
+        public void TestKitConfigTest()
+        {
+            using (var sys = ActorSystem.Create(nameof(TestKitSpec), TestKit.Xunit2.TestKit.DefaultConfig))
+            {
+                var testkit = new TestKit.Xunit2.TestKit(sys, _output);
+                var echoActor = testkit.Sys.ActorOf(c => c.ReceiveAny((m, ctx) => testkit.TestActor.Tell(m))); 
+                Invoking(() =>
+                {
+                    echoActor.Tell("message");
+                    var message = testkit.ExpectMsg<string>();
+                    message.Should().Be("message");
+                }).Should().NotThrow<ConfigurationException>();
+            }
+        }
+    }
+}

src/core/Akka.TestKit/TestKitBase.cs

@@ -147,6 +147,10 @@ protected virtual void InitializeTest(ActorSystem system, ActorSystemSetup confi
                 }
                 system = ActorSystem.Create(actorSystemName ?? "test", config.WithSetup(newBootstrap));
             }
+            else
+            {
+                system.Settings.InjectTopLevelFallback(_defaultConfig);
+            }
 
             _testState.System = system;