A repo dedicated to non-unit-test tests would be the best way to go. No need to pollute your main code repo with orders of magnitude more code and junk than the actual application.

That said, from what I understand of the exploit, it could have been avoided by having packaging and testing run in different environments (I could be wrong here, I've only given the explanation a cursory look). The tests modified the code that got released. Tests rightly shouldn't be constrained by other demands (like specific versions of libraries that may be shared between the test and build steps, for example), and the deploy/build step shouldn't have to work around whatever side effects the tests might create. Containers are easy to spin up.

Keeping them separate helps. Sure, you could do folders on the same repo, but test repos are usually huge compared to code repos (in my experience) and it's nicer to work with a repo that keeps its focus tight.

It's comically dumb to assume all tests are equal and should absolutely live in the same repo as the code they test, when writing tests that function multiple codebases is trivial, necessary, and ubiquitous.