test: fix flaky test-string-decode.js on x86

Removes flakiness from the mentioned test due to the x86 memory limit

PR-URL: #48750
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Debadree Chatterjee <debadree333@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

Author: StefanStojanovic

test/parallel/test-string-decoder.js

@@ -203,7 +203,7 @@ assert.throws(
 
 if (common.enoughTestMem) {
   assert.throws(
-    () => new StringDecoder().write(Buffer.alloc(0x1fffffe8 + 1).fill('a')),
+    () => new StringDecoder().write(Buffer.alloc((process.arch === 'ia32' ? 0x18ffffe8 : 0x1fffffe8) + 1).fill('a')),
     {
       code: 'ERR_STRING_TOO_LONG',
     }