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

The test is timing out on FreeBSD 10 in CI. It takes less than half as
long to run when it is in sequential on that platform instead of
parallel.

Refs: https://github.com/nodejs/node/pull/21322#issuecomment-399311718

PR-URL: https://github.com/nodejs/node/pull/21457


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>