Fix test case.
The specific problem was that the AST was looking for Actor/AnyActor in
_Concurrency... but I named the module of the test borrowing (for some reason).
So the machinery was failing to think that my stubbed out protocols where the
true known protocols. By changing the module name to _Concurrency, everything
worked out.
(cherry picked from commit 4aea60c4b14cc959bc84508c2773553e9f060ca6)