builtins: tests for verifying consistency

[DFrenkel]

Following open-policy-agent/opa@6415ef1, we add equivalent compliance tests to Swift OPA
compliance suite to verify consistent behavior
of uuid.rfc4122 and time.now_ns builtins
when invoked multiple times during a single eval.

[DFrenkel]

Note that consistent behavior for uuid.rfc4122 and time.now_ns invocations is enabled by plumbing BuiltinsCache and creation timestamp to all BuiltinContext instances at the moment the evaluate is called. The cache part is addressed in #33, and creation timestamp is addressed in #5, so this PR can either be merged with an expected/new compliance test failures related to caching or we wait on PRs mentioned here to be merged first. @shomron @patrick-east FYI

[srenatus]

Would it perhaps be a good moment to pull in all the current tests from OPA @ main? We're currently on some old snapshot converted with an internal tool; instead of adding a single file on top of that, I think we should fast-forward our snapshot... 🤔

[DFrenkel]

Would it perhaps be a good moment to pull in all the current tests from OPA @ main? We're currently on some old snapshot converted with an internal tool; instead of adding a single file on top of that, I think we should fast-forward our snapshot... 🤔

Yes, agreed. I'll mark this PR as draft and refresh all compliance tests from OPA @ main.