With timers you must make sure you connect the signals to the nodes where you want the functions called. If you made the timer in the editor, you will need to do this under the sub tab "node (signals)". If you did it in code, you will need to connect the signal.
I recommend doing something in your code above as below. You want to instance the slime in your timeout function. Or have an object pool of slimes where they are all instanced at the beginning of the scene and then they are woken up and placed in the timeout function.
const slime_scene = preload("res://slime.tscn")
print("Timer is up!")
slime = slime_scene.instance()