selftests: net: add test for variable PMTU in broadcast routes

Added a test for variable PMTU in broadcast routes.

This test uses iputils' ping and attempts to send a ping between
two peers, which should result in a regular echo reply.

This test will fail when the receiving peer does not receive the echo
request due to a lack of packet fragmentation.

Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
Link: https://patch.msgid.link/20250710142714.12986-2-oscmaes92@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>