test: wait for stream close before writing to file

Wait for async operations on a file to finish before writing to it
again.

This fixes flakiness in parallel/test-readline-async-iterators-destroy.

PR-URL: https://github.com/nodejs/node/pull/30836
Fixes: https://github.com/nodejs/node/issues/30660


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>