test: fix unreliable test-fs-write-file.js

The test uses a file name twice, causing unreliability in CI. In
particular, it's failing a lot on the Raspberry Pi devices.

Fixes: https://github.com/nodejs/node/issues/36090

PR-URL: https://github.com/nodejs/node/pull/36102


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>