test: enable more case of bad buffer in fs.write

Passing to the `string` parameter an object with an own
`toString` function is no longer supported.(DEP0162)
So use such case as bad buffer in test.

Refs: https://github.com/nodejs/node/blob/main/doc/api/deprecations.md#dep0162-fswrite-fswritefilesync-coercion-to-string
PR-URL: https://github.com/nodejs/node/pull/46236
Refs: https://github.com/nodejs/node/pull/42796
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-fs-write-optional-params.js

@@ -68,9 +68,7 @@ async function runTests(fd) {
     new Date(),
     new String('notPrimitive'),
     { [Symbol.toPrimitive]: (hint) => 'amObject' },
-
-    // TODO(LiviaMedeiros): add the following after DEP0162 EOL
-    // { toString() { return 'amObject'; } },
+    { toString() { return 'amObject'; } },
   ]) {
     await testInvalid(fd, 'ERR_INVALID_ARG_TYPE', badBuffer, {});
   }