Add an end-to-end correctness test for ckks.relinearize

This issue is for the task of testing the lowerings of `ckks.relinearize` end-to-end, via lowering the ops to LLVM and running a mul + relinearize on ciphertexts.

https://github.com/google/heir/issues/2987 identified a bug in the implementation of basis conversion that this would have caught.

The first step should be scoping the work required to complete this, because I think there are still a few nontrivial gaps. For example, I don't think we have any ops (not to mention lowerings) for generating relinearization keys.

CC @crockeea @bencemali @AlexanderViand