If LLMs are good enough to write code, they are good enough to write tests. In fact, they can be quite creative at finding test scenarios which exercise the code in unexpected ways.
The usefulness of testing is basically orthogonal to the advent of language models. This question boils down to whether you think the agents are "cheating" through changing the invariants encoded by the tests to something that no longer matches your project's requirements. To this, I would suggest that you read the code they produce, tests included, and not sign off on stupid changes to cheat on tests. This is not a common thing I've caught agents doing, FWIW.
The usefulness of testing is basically orthogonal to the advent of language models. This question boils down to whether you think the agents are "cheating" through changing the invariants encoded by the tests to something that no longer matches your project's requirements. To this, I would suggest that you read the code they produce, tests included, and not sign off on stupid changes to cheat on tests. This is not a common thing I've caught agents doing, FWIW.