testsuite: split protocol-specific code out of testTransparentWithPID

Extract TCP dial/send/close into transparentTCPDialAndSend callback
so that testTransparentWithPID is protocol-generic and can be reused
for a future TestUDPTransparent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>