By "reduced need for tests" I didn't mean that the absolute number of tests would decline, but rather the need and incentives for the development team to write the tests themselves. Since they have the ecosystem providing tests for them, they don't need to make as many themselves. At least, that's how I understood the discussion.
So yes, if the package manager only enforced the tests you include in your package it would incorrectly discourage including tests. But if it enforces tests that _other_ people provide, you have no way around it. The only problem is how to handle bad tests submitted by other people. Maybe only enforce tests that passed on a previous version but fail on the current candidate?
Ooh, that's another novel idea I hadn't considered. I don't know how I feel about others being able to add to my automated test suite, though. Would one of my users be able to delete tests that another wrote? If they only have create privileges, not update, how would they modify tests? Who has the modification privileges?
These are whole new vistas, and it seems fun to think through various scenarios.
It's not really the same as others being able to add tests to your automated suite. Rather, they add tests to their own package, and then the CI tool collects all tests indirectly dependent on your library into a virtual suite. Those tests are written to test their code, and only indirectly test yours. If a version of their package passes all of their tests with a previous version of your code, but the atomic change to the latest version of your code causes their test to fail, the failure was presumably caused by that change. The tests will probably have to be run multiple times to eliminate non-determinism.
It's still possible that someone writes code that depends on "features" that you consider to be bugs, or a pathologically sensitive test, so there may need to be some ability as the maintainer to flag tests as poor or unhelpful so they can be ignored in the future. Hopefully the requirement that the test pass the previous version to be considered is sufficient to cover most faulty tests though.