test: fix test-net-throttle

Repeat writes until data is queued in memory, rather than assuming that
it will happen by a certain point.

Fixes: https://github.com/nodejs/node/issues/33135

PR-URL: https://github.com/nodejs/node/pull/33329


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>