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
Use
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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