I like the idea, but in reading up on time_scale, I think it has too many caveats to manage this in GUT. As soon as we add a wrapper, users tend to think "you let me do this, why aren't you protecting me from myself?" I'd want to know a ton more about how things can go wrong before making this a feature of GUT.

Maybe a Tips/Tricks page would be a good place to start. We could put in an example FastTest class with a lot of warnings and links to Godot docs. Maybe a call-to-action to give feedback if they have any success with this approach.

class_name MyFastTest
extends GutTest
var default_time_scale := Engine.get_time_scale() # or just 1.0
# add in physics and maybe audio ticks as well.
var fast_scale := 2.0

func before_each():
   # Having a way to disable fast tests is probably a good idea.
   if(MyGlobalsSomewhere.use_fast_tests):   
      Engine.set_time_scale(fast_ticks)

func after_each():
   Engine.set_time_scale(default_time_scale)

func after_all():
   Engine.set_time_scale(default_time_scale)

extends MyFastTest

func before_all():
    fast_scale = 4.5

func before_each():
   super()
   ...
func after_each():
   super()
   ...
func after_all():
   super()
   ...

<example tests>

This might also be good place to reference the hooks described here (but not yet added to the documentation). Being able to also reset the scales after a script has finished running (just in case something goes really wrong in a test script) would be good.

I'm most concerned about what it looks like to keep time_scale and physics_ticks_per_second synced to reasonable values across different platforms/environments. Especially when CI/CD pipelines are involved. This could all work fine locally but the pipeline becomes very flakey.

Note: It's recommended to keep this property above 0.0, as the game may behave unexpectedly otherwise.

Note: This does not affect audio playback speed. Use AudioServer.playback_speed_scale to adjust audio playback speed independently of Engine.time_scale.

Note: This does not automatically adjust physics_ticks_per_second. With values above 1.0 physics simulation may become less precise, as each physics tick will stretch over a larger period of engine time. If you're modifying Engine.time_scale to speed up simulation by a large factor, consider also increasing physics_ticks_per_second to make the simulation more reliable.