Merge branch 'add-selftests-helper-to-get-n-unique-ports'

Dimitri Daskalakis says:

====================
Add selftests helper to get N unique ports

The rss_ctx.py tests would occasionally flake. I found that the successive
calls to rand_port would occasionally return duplicate ports, breaking the
tests invariants.

Add a new helper that guarantees generated ports are unique.
====================

Link: https://patch.msgid.link/20260224224659.1507082-1-dimitri.daskalakis1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>