Consider moving test modules out of system module, unless they relate to system.module

Pushed up a proof of concept, needs review for the idea.

+1 for the idea

I was literally just thinking about how to better organize all of the hook related test modules.

Since the substantive change is to the discovery I think I'll change the component.

Added complete steps to move all the identified modules if we decide to proceed with this.

Would it be possible to create subdirectories inside core/tests/modules? At least for components with many test modules (e.g., hooks).

Yes this should detect subdirectories

Unfortunately this is postponed on: #3067979: Exclude test files from release packages →

@nicxvan can you explain why, or point to the discussion where that decision was made? I would have thought that issue would be a simple wildcard pattern somewhere and this would not disrupt that.

I have to find the reference, maybe we can get adding new ones there, but comment 63 in the linked issue is the main issue, breaking patches just for moving test modules is major only.

If we split the move from support we can add support here.

I found the discussion in slack: https://drupal.slack.com/archives/C079NQPQUEN/p1752356974306669

Can we remove the module moves here and do a quick performance test with this change?
I think we can add the new discovery, but since it's for tests only I wouldn't want it to impact actual discovery, I think it will be fine though.

That slack message means we have to wait for the 12.x release for excluding tests from patches for the actual move, but we can still add support for new modules.