Establish "core/tests/$type" as a canonical place for test extensions

I think this needs some kind of test, but not sure yet how.
We can add a proof-of-concept module and have a test to discover it.
Will this be enough? Is it necessary?

Instead of a dedicated test, we could use the two issues I mentioned as proof-of-concept.
If these work, we know that the change is ok.

I love the idea! If committers agree think we should open a follow up to move the existing modules to the new directory.

move the existing modules to the new directory.

Just as a note:
Currently the modules in core/tests/Drupal/Tests/Core/Extension/modules/, in core/tests/Drupal/Tests/Core/Database/fixtures/core/modules/ and in core/tests/fixtures/ are not found by ExtensionDiscovery.
They are used for unit tests where we have to manually add these paths.
Moving them to a place where they are discovered _could_ have side effects that we need to watch out for.
I don't see any name clashes with test modules in core/modules/**/tests/, so I guess it will be fine.

I think this addition should have either testing topic maintainer or FM review (tagged both, but really just one or the other would do). Thanks!

This looks reasonable to me, and is consistent with how we store test modules for core modules.

I have one concern regarding the weight, specifically that we have core/modules at weight 0, ie explicitly the lowest in the priority. This is for example to allow a contrib version of a core module (e.g. CKEditor4, RDF) to take precedence over core when both exist (such as towards the end of Drupal 9).

Adding this new directory at weight 6 means it would take precedence over the other locations. I think instead it should probably be given weight -1 and added before the core directory. So that it gets the lowest precedence.

But in terms of the concept, I think this makes sense.

Is there an existing module in core/tests/Drupal/Tests/Core/Extension/modules/ we can move as part of this issue to provide implicit test coverage?

This will also need a change record and a followup to move any other existing modules.