Introduce an app::run_system_set() function

[cBournhonesque]

What problem does this solve or what need does it fill?

For testing or benchmarking purposes, I would like to run a bunch of systems of my App that are all in the same SystemSet.
Bevy currently provides a function app.run_schedule(schedule) and app.run_system() (for registered one-shot systems) but there is no helper to run an entire system set.

What solution would you like?

Have an api app.run_system_set(schedule_name, system_set_name) which runs all the systems inside a SystemSet once.

What alternative(s) have you considered?

I could try to put all those systems in a separate Schedule, but:

• from what I understand, two schedules cannot run in parallel, and I would like those systems to run in parallel with other PostUpdate systems
• it would be a big refactoring of my app to just gain some testing capabilities

[alice-i-cecile]

I'm on board with this feature. It's calling convention should mirror the run_schedule API.

[ItsDoot]

How would this work with regards to individual systems that have ordering constraints outside the specified system set? Should they be ignored, or should it include all of them? Currently feels like a footgun to me when app.run_schedule() is an option.

[cBournhonesque]

It would just skip any system that is not in the SystemSet.
It seems strange to me that i'm forced to create a different schedule just to test run a subpart of my app