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

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Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines