test(persist): assert the open test transaction and the optional commit_ids result

[wolfy-j]

The persist tests read a transaction that before_each opens and after_each resets to nil, and a commit_ids field the commit result sets only when commits were applied. The go-lua type checker fixes in wippyai/go-lua#51 now see both as possibly nil (29 errors in the module's lint). The helpers assert what the tests rely on.

Test-only; no runtime behavior change.