Make InternalTestActor override its SupervisionStrategy

[yanpitangui]

Fixes #7062

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[Aaronontheweb]

LGTM but you should add a very simple regression test to verify that this works, mostly to protect us if we make some changes to the test suite in the future. Does that make sense @yanpitangui ?

[Aaronontheweb]

Aha, looks like we already have a test that touches this and it's now failing as a result of this change: Akka.TestKit.Tests.TestActorRefTests.TestProbeSpec.TestProbe_restart_a_failing_child_if_the_given_supervisor_says_so

my guess is that the test is also wrong - so maybe if we just fix that we should be in good shape @yanpitangui