TST: Add back pytest-filter-subpackage - #20165

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Yes, I think we should be practical here. Removing it unceremoniously in #18784 and making core maintainers unhappy is not going to help us retain maintainers. 😉

I have to admit I still don't remember how to install dependency groups, after doing pip install .[test] for 15 years it's a hard habit to break 😆

@neutrinoceros you okay with merging this? Thanks, all!

I still don't remember how to install dependency groups

--group <name-of-the-group> it's actually more discoverable than "extras" requiring special syntax