Don't use GlobalScope to launch flow and add cleanup method

[sockeqwe]

Let's create our own Scope and add a cleanup method to cancel the scope and release all recorded emissions.

[sockeqwe]

Added a .cleanUp() method and updated docs.

No now creating our own scope because one can pass its own scope as constructor parameter. If we would internally close the scope in cleanUp() we might close a scope that is used somewhere else too. To avoid that we don't introduce our own scope for now and just close the channels and Jobs used internally instead of going with closing a coroutine scope.

Another option is to could create our own coroutine scope and use it internally but don't allow consumers of this library to pass their own coroutine scope to launch the flow under test in it.

For now we go with the first option.