Looking at the web, I can find old memories [1] where postgresql12-devel installed them in /usr/pgsql-12/lib/pgxs/src/test/perl/
We really need some consistency and cross checks between the RPMs and debs.
Ah. I just noticed you said they're installed with the *client* packages.
So that's an example of how we actually need some postgres packaging guidance that applies to rpms, debs, EDB's binary installers, and anything else, separate to any particular packaging method or team.
In this case I think it's a bit silly to put the test perl modules along with the client. They can't really be used without the server installed, and they're not useful for much unless you're building extensions. They should be in a -dev package IMO. They should preferably not be in the test package along with the actual test input files, though, since they're used by extensions to run their own tests and extensions only have to depend on server-dev not on a -test package.
Anyway, I hope the above answers your questions about where they live.
