Rework FakeSignalManager tests to fix flakiness

[Jeehut]

During the migration to Swift testing, I noticed that the tests for the FakeSignalManager have become flaky. Each time I run tests, other tests pass and others fail. Sometimes all pass. But this is not a situation we can keep as is. Because no logic was changed (neither in the tests nor in the related code) and the tests sometimes all pass, I'm assuming that there is no actual problem with the library and decided to disable them for now. The problem most probably lies in the way the SDK uses different dispatch queues in different places and Swift 6's concurrency logic not playing well with that in Swift Testing.

So, I think we need to rework how the whole Signal manager testing logic is implemented.