Making the work in #1013 more manageable this is to propose doing KAT tests against the low-level crypto API for MLKEM separately. All work obtaining test vectors, running tests and confirming correctness, e.g., JSON parsing etc. could be done as part of resolving this task and then be re-used in #1013 when testing against the EVP API, thus validating the whole integration.

This is a reversal of my previous position (to do KAT testing only at the EVP level to be efficient) in recognition of the need to split work onto more shoulders.