+tes #21198 Allow TestActor/TestKit/TestProbe to create child actors

This is useful in at least two scenarios:
- Unit testing actors that communicate to their parent directly
- Testing re-creating (typically persistent) actors with the same name

akka-docs/rst/scala/code/docs/testkit/ParentChildSpec.scala

@@ -121,6 +121,17 @@ class ParentChildSpec extends WordSpec with Matchers with TestKitBase with Befor
     }
   }
 
+  //#test-TestProbe-parent
+  "A TestProbe serving as parent" should {
+    "test its child responses" in {
+      val parent = TestProbe()
+      val child = parent.childActorOf(Props[Child])
+      parent.send(child, "ping")
+      parent.expectMsg("pong")
+    }
+  }
+  //#test-TestProbe-parent
+
   //#test-fabricated-parent
   "A fabricated parent" should {
     "test its child responses" in {

akka-docs/rst/scala/testing.rst

@@ -548,26 +548,51 @@ Testing parent-child relationships
 ----------------------------------
 
 The parent of an actor is always the actor that created it. At times this leads to
-a coupling between the two that may not be straightforward to test. 
-Broadly, there are three approaches to improve testability of parent-child 
-relationships: 
+a coupling between the two that may not be straightforward to test.
+There are several approaches to improve testability of a child actor that
+needs to refer to its parent:
 
-1. when creating a child, pass an explicit reference to its parent 
-2. when creating a parent, tell the parent how to create its child
+1. when creating a child, pass an explicit reference to its parent
+2. create the child with a ``TestProbe`` as parent
 3. create a fabricated parent when testing
 
+Conversely, a parent's binding to its child can be lessened as follows:
+
+4. when creating a parent, tell the parent how to create its child
+
 For example, the structure of the code you want to test may follow this pattern: 
 
 .. includecode:: code/docs/testkit/ParentChildSpec.scala#test-example
 
-Using dependency-injection
-^^^^^^^^^^^^^^^^^^^^^^^^^^
+Introduce child to its parent
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The first option is to avoid use of the :meth:`context.parent` function and create 
 a child with a custom parent by passing an explicit reference to its parent instead.
 
 .. includecode:: code/docs/testkit/ParentChildSpec.scala#test-dependentchild
 
+Create the child using TestProbe
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``TestProbe`` class can in fact create actors that will run with the test probe as parent.
+This will cause any messages the the child actor sends to `context.parent` to
+end up in the test probe.
+
+.. includecode:: code/docs/testkit/ParentChildSpec.scala##test-TestProbe-parent
+
+Using a fabricated parent
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you prefer to avoid modifying the parent or child constructor you can 
+create a fabricated parent in your test. This, however, does not enable you to test
+the parent actor in isolation.
+
+.. includecode:: code/docs/testkit/ParentChildSpec.scala#test-fabricated-parent
+
+Externalize child making from the parent
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 Alternatively, you can tell the parent how to create its child. There are two ways 
 to do this: by giving it a :class:`Props` object or by giving it a function which takes care of creating the child actor:
 
@@ -581,14 +606,6 @@ And like this in your application code:
 
 .. includecode:: code/docs/testkit/ParentChildSpec.scala#child-maker-prod
 
-Using a fabricated parent
-^^^^^^^^^^^^^^^^^^^^^^^^^
-
-If you prefer to avoid modifying the parent or child constructor you can 
-create a fabricated parent in your test. This, however, does not enable you to test
-the parent actor in isolation.
-
-.. includecode:: code/docs/testkit/ParentChildSpec.scala#test-fabricated-parent
 
 Which of these methods is the best depends on what is most important to test. The
 most generic option is to create the parent actor by passing it a function that is