test: update test-fs-fsync to run from temp

We copy the "a.js" file fixture to a temp directory and then we open
it with write access. This makes sure we are only writing to the
provided temp-dir and not in the fixture dir.

PR-URL: https://github.com/nodejs/node/pull/15537


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>