Allow to explicitly opt-out a settings form from domain_config.

Do you mean that you simply want to disable the "Enable domain configuration" button for that particular form?

Yes, exactly.

Looks like an easy fix to me.

The simplest would be to add a hook to allow custom modules to populate that list of forbidden paths.

We could also add a new configuration field allowing to define the list of forbidden form paths.

I have implemented a hook that allows other modules to add forbidden paths for config_ui.
I also implemented a test for the new functionality.

Please review.

I have changed the hook to be _info, so modules can return their forbidden paths, without interacting with the paths returned by other modules.

Regarding the way of checking if the "Enable domain configuration" is present on the page, i have reused the same strategy from other tests in the module, which do it that way already.

You are right @mably, that check did not make much sense. I changed the test to check if the "Enable domain configuration" link is not present instead.