Reduce the overall warnings from tests

[phausler]

This resolves almost all of the warnings in the tests (except one).

[kabiroberai]

👋 hey @phausler, you might wanna have a look at swiftlang/swift#66934 which should fix the deprecation warning (given that there's not much you can do about it pre-stdlib-5.9). That said, it's worth noting that, iiuc, the ExecutorJob overload here will never be used since the UnownedJob variant is preferred. If my PR lands, it should hopefully be possible for you to remove the former until pre-5.9 platforms are dropped — or maybe gate it behind a !canImport(Darwin) conditional.