test: fix flaky test-webcrypto-encrypt-decrypt-aes

* Use a copy of plaintext to prevent tampering of the original
* Since subtle.decrypt returns a Promise containing an ArrayBuffer and
  ArrayBuffers cannot be modified directly, create a Buffer from it
  right away so that the modification in the next line works as intended

Fixes: https://github.com/nodejs/node/issues/35586

PR-URL: https://github.com/nodejs/node/pull/37380


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>