ActorTestKit uses @testable import DistributedCluster, causing problems for build -c release

[ktoso]

The testkit inherently DOES pry into a lot of the actor runtime internals of the cluster. It provides mocks and test probes and various things that pretend to be behavior actors but are not.

This needs to use internal APIs, and we do so by using a @testable import in the TestKit.
Making it not need that is a painful task making a lot of "old runtime" types public, which we absolutely don't want to do, given that now actors and distributed actors are in the language, so we would not want to expose MORE of our previous infrastructure.

If we had the package access visibility this would solve the issue. https://forums.swift.org/t/se-0386-package-access-modifier/62808/

This prevents the source-compatibility-suite to build the cluster in release mode.

[ktoso]

Related issue rdar://106086239

[ktoso]

Ah, we can move to @_spi(ActorTestKit) and that should work, excellent