feat: fallback config for testkit actor system (#2377)

* feat: fallback config for testkit actor system

* fmt

Author: aludwiko

sdk/java-sdk-protobuf-testkit/src/main/java/kalix/javasdk/testkit/KalixTestKit.java

@@ -636,7 +636,8 @@ public KalixTestKit start(final Config config) {
     testSystem =
         ActorSystem.create(
             "KalixTestkit",
-            ConfigFactory.parseString("akka.http.server.preview.enable-http2 = true"));
+            ConfigFactory.parseString("akka.http.server.preview.enable-http2 = true")
+                .withFallback(config));
 
     int eventingBackendPort = startEventingTestkit(useTestContainers);
     runProxy(useTestContainers, port, eventingBackendPort);