gh-91162: Fix most generic type substitution test cases #92427
                
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@serhiy-storchaka Darn, I'd just finished this when I realised you'd also been working on the same thing in main...serhiy-storchaka:typing-subst-unpacked. I think the version in my branch uses more helper functions in a way that improves readability, but your version does things in a way that's more consistent with the new design of typing.py - and of course fixes a lot of the C stuff that my branch doesn't touch. How do you want to proceed here?
Quick summary of what's in this PR: