test: use valid authentication tag length

Using authentication tags of invalid length does not conform to NIST
standards.

PR-URL: https://github.com/nodejs/node/pull/17566
Refs: https://github.com/nodejs/node/issues/17523


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>