test: add string-decoder fuzz test

PR-URL: https://github.com/nodejs/node/pull/22709
Fixes: https://github.com/nodejs/node/issues/22626


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>