> The LLM's tests passed, which is fine and dandy, but how do I know the tests are truly valid if I do not understand what is fundamentally being tested?

Do what I do sometimes: use opaque pointers (in C, or C++), and don't give the LLM the implementation, give them only the header, when asking it for tests.

I've lost count of the number of times I'll give only the header, but the LLM insists it needs to see the implementation as well in order to write the tests.

If you're not using C or C++, well, then write interfaces, give that to the LLM.