test: do not open fixture files for writing

Use temp directory for open with `O_DSYNC` (which indicates a write may
occur) rather than `fixtures` directory. Additionally, test can be run
on macOS so allow that in addition to Linux.

PR-URL: https://github.com/nodejs/node/pull/17810


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>