test: do not open fixture files for writing

test-fs-fsync makes a copy of a fixture file, but then doesn't do
anything with it and instead operates on the original fixture file. This
appears to be in error, and this change fixes that.

PR-URL: https://github.com/nodejs/node/pull/17808


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
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>