Compile akka-actor-testkit with scala 3 (#30395)

Interestingly the Java code in the tests now doesn't compile, so something
around generics might still be wrong.

.travis.yml

@@ -38,7 +38,7 @@ jobs:
     - stage: scala3
       name: scala3
       # separate job since only a few modules compile with Scala 3 yet
-      script: jabba install adopt@1.11-0 && jabba use adopt@1.11-0 && sbt -Dakka.build.scalaVersion=3.0 "akka-actor-tests/test:compile" akka-actor-typed/compile akka-pki/test:compile akka-protobuf/test:compile akka-protobuf-v3/test:compile akka-slf4j/test:compile akka-stream/compile akka-stream-tests-tck/test
+      script: jabba install adopt@1.11-0 && jabba use adopt@1.11-0 && sbt -Dakka.build.scalaVersion=3.0 "akka-actor-tests/test:compile" akka-actor-testkit-typed/compile akka-actor-typed/compile akka-pki/test:compile akka-protobuf/test:compile akka-protobuf-v3/test:compile akka-slf4j/test:compile akka-stream/compile akka-stream-tests-tck/test
 
 stages:
   - name: whitesource

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/TestKitSettings.scala

@@ -44,7 +44,7 @@ object TestKitSettings {
 
   private object Ext extends ExtensionId[Ext] {
     override def createExtension(system: ActorSystem[_]): Ext = new Ext(system)
-    def get(system: ActorSystem[_]): Ext = apply(system)
+    def get(system: ActorSystem[_]): Ext = Ext.apply(system)
   }
 
   private class Ext(system: ActorSystem[_]) extends Extension {

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/BehaviorTestKitImpl.scala

@@ -30,7 +30,7 @@ private[akka] final class BehaviorTestKitImpl[T](_path: ActorPath, _initialBehav
     with akka.actor.testkit.typed.scaladsl.BehaviorTestKit[T] {
 
   // really this should be private, make so when we port out tests that need it
-  private[akka] val context = new EffectfulActorContext[T](_path, () => currentBehavior)
+  private[akka] val context: EffectfulActorContext[T] = new EffectfulActorContext[T](_path, () => currentBehavior)
 
   private[akka] def as[U]: BehaviorTestKitImpl[U] = this.asInstanceOf[BehaviorTestKitImpl[U]]
 

akka-actor-testkit-typed/src/main/scala/akka/actor/testkit/typed/internal/StubbedActorContext.scala

@@ -76,7 +76,7 @@ private[akka] final class FunctionRef[-T](override val path: ActorPath, send: (T
   @InternalApi private[akka] val selfInbox = new TestInboxImpl[T](path)
 
   override val self = selfInbox.ref
-  override val system = new ActorSystemStub("StubbedActorContext")
+  override val system: ActorSystemStub = new ActorSystemStub("StubbedActorContext")
   private var _children = TreeMap.empty[String, BehaviorTestKitImpl[_]]
   private val childName = Iterator.from(0).map(Helpers.base64(_))
   private val substituteLoggerFactory = new SubstituteLoggerFactory