Fix MessageAdapterSpec instability

[raboof]

Consume dead letters in one test so they don't
pollute the next test

Fixes #30552

[raboof]

Hmm, the failure is actually in the test that was touched in this PR, weird that I didn't see this locally:

must not catch exception thrown after adapter, when processing the message *** FAILED *** (24 milliseconds)
[info]   Pong(hello) was not equal to Pong(hello) (MessageAdapterSpec.scala:273)
[info]   org.scalatest.exceptions.TestFailedException:
[info]   at org.scalatest.matchers.MatchersHelper$.indicateFailure(MatchersHelper.scala:402)
[info]   at org.scalatest.matchers.should.Matchers.shouldBe(Matchers.scala:6985)
[info]   at org.scalatest.matchers.should.Matchers.shouldBe$(Matchers.scala:1808)
[info]   at akka.actor.testkit.typed.scaladsl.ScalaTestWithActorTestKit.shouldBe(ScalaTestWithActorTestKit.scala:31)
[info]   at akka.actor.typed.scaladsl.MessageAdapterSpec.f$proxy5$2(MessageAdapterSpec.scala:273)
[info]   at 

[raboof]

(mima failures are fixed with #30665 )