test: add flag scenario in test-fs-write-file-sync

fs.writeFileSync takes flag param to define the file opening
semantics. Add a scenario that covers flags as well.

PR-URL: https://github.com/nodejs/node/pull/24766


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>