Items that are necessary for building and testing this distribution, but aren't necessary after installation. This can help users who only want to install these items temporarily. It also helps reduce the size of the CPAN dependency graph if everything isn't smooshed into requires.
Yeah, it could use a 'build_recommends' and a 'configure_recommends' section. But it sounds like even that isn't what you want, really.
Personally, I'd say if you really don't expect people to run those tests except when in development, I'd just not put them in the Build.PL or Meta.yml: They aren't part of the user's installation. They are part of your development suite. A programmer can find them in the test code, if they need to see them. (And they'll know to look because you are skipping the tests with a good message. Right?)
Some people even have an environment variable they check for this case, and don't even try to run the tests if it isn't there. I've never quite gotten that complicated myself...