Rollup merge of #136635 - jieyouxu:base_port, r=joboet

Remove outdated `base_port` calculation in std net test

This was never modified since `std::net` was originally introduced in 395709ca6d39ba1e095e404e1d2a169d918b7f0c, when at that time, each CI runner was running multiple jobs concurrently. This seems to have originally caused issues with jobs fighting over the same ports. This is not the case in the current CI infrastructure, so remove this relic in favor of a simple constant base port number.

I double-checked `19600` and nearby port numbers, and this isn't a well-known port number AFAICT[^1].

Closes #136633.

[^1]: At the time of writing.

Author: matthiaskrgr