test: move test-net-throttle to parallel

The test is not very resource intensive. It just writes data to a TCP
socket until backpressure is reached. Move it parallel.

PR-URL: #48599
Fixes: #40507
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Jacob Smith <jacob@frende.me>
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Nitzan Uziely <linkgoron@gmail.com>