Fix keywrap response size checks

[padelsbach]

And add unit tests.

Fixes F-3104

[padelsbach]

New test case might be overkill for these simple fixes -- tried to keep them as simple as possible...

[bigbrett]

hmm I also dont like the idea of having fake transport spoofing style tests just to ensure we are checking args correctly. Does smell like overkill, at least for now.

Lets phrase the problem in a different manner - assume we want to eventually get to 100% code coverage...given the new test framework, how would we structure it such that the higher level behavioral integration tests can be somewhat isolated from the API/unit/spammy style tests that just ensure "assert" behavior on args.

Does it make sense to have a totally separate group of "API/arg" tests, also organized by module?

I know this is overkill to discuss this for such a small PR but I'd rather iron these details out up front as we are integrating.

Thoughts?