test: move net bytes-per-chunk test to parallel

This was added in 3217e8e as
a regression test for a security patch. We moved it to
`sequential` to lower the risk of creating a flaky test,
because an earlier version of it was failing one some platforms.

There is no known reason why te test should be flaky in this form,
though, and moving it to parallel would be good because it does take
around 3 seconds that would otherwise fully count towards the
test run time.

PR-URL: #21322
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>