Add TCP loopback test infrastructure to nettest

Add TCP loopback test infrastructure to nettest: a task in LegoFlow-SWE (Harbor dataset). The network testing framework ( nettest ) currently has no test coverage for TCP connections. We need to add test cases that verify TCP loopback communication over both IPv4 and IPv6, using a shared…

The task

The network testing framework (`nettest`) currently has no test coverage for TCP connections. We need to add test cases that verify TCP loopback communication over both IPv4 and IPv6, using a shared background echo server that manages a TCP listening socket on the loopback interface.

Part of Lego-X/LegoFlow-SWE.