Eliminate `sleep` calls from tests

The unit tests currently use a few calls to `sleep` with very small waits in order to give control to the thread.

These calls don't hurt "much" to the test time because the few tests we have run in a couple seconds anyway. However, it probably is a smell of a lack of understanding of how we interface with the gem.

For example, if the gem was a regular `Thread`, the proper way of handling control to it would be to `wait`. However, the gem it's not really a thread, so this is not possible.

The point of this task is to go in that direction: think (and implement) a way to test `UniqueThread` in a similar way that one would deal with a regular `Thread`