test: increase limit for network space overhead test

This test imposes a limit on the average bytes of space per chunk
for network traffic. However this number depends on VM
implementation details, and upcoming changes to V8's
array buffer management require a small bump to this
limit in this test.

PR-URL: https://github.com/nodejs/node/pull/28492


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>