Fix .replay(1) when only one value has been sent on the HotSignal

It took me a while to track this down, but it looks like `.replay(1)` on a hot signal would result in an empty `ColdSignal` if the hot signal had only ever sent one value.

/cc @jspahrsummers 
That'll re-define the variables inside the beforeEach scope unfortunately. I tried doing `                (signal, sink) = HotSignal<Int>.pipe()` but the compiler complained.

Do you know if I can assign that way while keeping `signal` and `sink` defined in the outer block?