test: save test file in temporary directory

The readv_sync.txt test file is currenly saved in the root directory.
Save it in the dedicated temporary directory.

PR-URL: https://github.com/nodejs/node/pull/32670


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>