Make BehaviorTestKit message adaptation more synchronous

[leviramsey]

(Taking from a discussion in #30050)

As it stands, when using the BehaviorTestKit to test a behavior which performs message adaptation, testing the adaptation requires a call to testkit.runOne() to process the adaptation.

// From BehaviorTestKitSpec.scala
adaptorRef ! 2
testkit.selfInbox().hasMessages should be(true)
testkit.runOne()

The message in the selfInbox is not inspectable and this arguably exposes too much of the internal machinery.

#30050 plumbs the testkit through to the EffectfulActorContext, which allows the completion of the ask to synchronously run message adaptation in the behavior interceptor without going through the selfInbox. A similar approach can be taken with message adaptation to improve ergonomics in the common case.

As with the ask, if a behavior, in response to some message, sets up an adaptation and sends a message to itself, it would now be the responsibility of the test to runOne before sending to the adapter (which is in fact already implicitly the case: in the above test, if there was already a message in the selfInbox it would be run, not the adapted message).

If this change is made, it most likely could not be until 2.7/3.0, due to the potential test breakage.