handle base64 encoded dn attr

DN can be base64 encoded.

RFC 2849:

    base64-distinguishedName = BASE64-UTF8-STRING
                               ; a distinguishedName which has been base64
                               ; encoded (see note 10, below)
`assert ...` is equivalent to `assert_equal true, ...`, but also check out `assert_predicate`.

I see that this is just based off other tests in the file, though. I'm happy to take care of cleaning those up separately.