test: fix http-writable-true-after-close flakyness

Ref: https://github.com/nodejs/node/pull/15404
Fixes: https://github.com/nodejs/node/issues/15505
PR-URL: https://github.com/nodejs/node/pull/15520


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