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: #35586

PR-URL: #37380
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

test/parallel/parallel.status

@@ -25,8 +25,6 @@ test-worker-memory: PASS,FLAKY
 test-worker-message-port-transfer-terminate: PASS,FLAKY
 
 [$system==linux]
-# https://github.com/nodejs/node/issues/35586
-test-webcrypto-encrypt-decrypt-aes: PASS,FLAKY
 
 [$system==macos]
 

test/parallel/test-webcrypto-encrypt-decrypt-aes.js

@@ -9,6 +9,9 @@ const assert = require('assert');
 const { getRandomValues, subtle } = require('crypto').webcrypto;
 
 async function testEncrypt({ keyBuffer, algorithm, plaintext, result }) {
+  // Using a copy of plaintext to prevent tampering of the original
+  plaintext = Buffer.from(plaintext);
+
   const key = await subtle.importKey(
     'raw',
     keyBuffer,
@@ -23,8 +26,10 @@ async function testEncrypt({ keyBuffer, algorithm, plaintext, result }) {
     Buffer.from(output).toString('hex'),
     Buffer.from(result).toString('hex'));
 
-  const check = await subtle.decrypt(algorithm, key, output);
-  output[0] = 255 - output[0];
+  // Converting the returned ArrayBuffer into a Buffer right away,
+  // so that the next line works
+  const check = Buffer.from(await subtle.decrypt(algorithm, key, output));
+  check[0] = 255 - check[0];
 
   assert.strictEqual(
     Buffer.from(check).toString('hex'),