Sanity check: kadm5 unit test organization change

[ghudson@MIT.EDU]

I'd like to reorganize the kadm5 unit tests slightly in order to
reduce the amount of churn associated with bumping the kadmin API
version.

Right now we have directories api.1, api.2, and (recently added)
api.3.  This is good because it lets us test compatibility with older
versions of the API, but bad because we have to copy or move all of
the test cases every time we rev the API, no matter how minor the
change to the API.

What I'd like to do is have api.1, api.2, and api.current.  The next
time we bump the API version, we'll create api.3 and populate it with
the specific test cases whose behavior changes between version 3 and
version 4; the tests in api.current will remain in place and will just
be updated for the new API version.