tests/netinet: push variable 'sin' into function scope

This should fix build with GCC 13.

Fixes: 861274c9f8cf2a5c00ded9580ea91b22def683f7

tests/netinet: add a demo of TCP implied connect

The TCP implied connect is an artifact left after T/TCP. To my surprise
it still works, hence the existence of this test. Please read this email
first:

https://lists.freebsd.org/pipermail/freebsd-net/2010-August/026311.html

An interesting fact that this test takes 220 - 240 milliseconds to
execute on my Threadripper PRO. Flipping the '#if 0' to '#if 1' in the
test, thus bringing it back to normal connect(2), would speed the test up
a hundred times and I guess all this time is fork+exec of the test.