selftests: net: py: Add rand_ports helper method

selftests: net: py: Add rand_ports helper method

Certain tests need a unique set of ports. Successive calls to the
existing rand_port method may return a duplicate port, resulting in test
flakiness. The new helper keeps sockets open while building a list of
ephemeral ports, thus the kernel enforces their uniqueness.

Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Link: https://patch.msgid.link/20260224224659.1507082-2-dimitri.daskalakis1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>