+tes #16431 support for TestProbe with user-defined name

TestProbe can be given a user-defined name

```scala
val worker1 = TestProbe("worker1")
val worker2 = TestProbe("worker2")
```
we usually use this style:

```
.. includecode:: code/docs/testkit/TestKitDocTest.java#test-probe-with-custom-name
```