selftests: net: fix passive TFO test to fail if child processes failed

selftests: net: fix passive TFO test to fail if child processes failed

Improve the passive TFO test to report failure if the server or the
client timed out or exited with non-zero status.

Before this commit, TFO test didn't fail even if exit(EXIT_FAILURE) is
added to the first line of the run_server() and run_client() functions.

Signed-off-by: Yohei Kojima <yk@y-koj.net>
Link: https://patch.msgid.link/214d399caec2e5de7738ced5736829915d507e4e.1768312014.git.yk@y-koj.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>