sha2-256-96 test

hmac-sha2-256-96 was discussed in an draft of RFC6668:

http://tools.ietf.org/html/draft-dbider-sha2-mac-for-ssh-03

Later versions removed it, however. In light of it's removal phpseclib will not officially support hmac-sha2-256-96. This branch is only to test that it works so that https://github.com/phpseclib/phpseclib/pull/446 might be merged