Merge branch 'selftests-net-py-improve-bkg-error-reporting'

Jakub Kicinski says:

====================
selftests: net: py: improve bkg() error reporting

bkg() is a helper for running commands in the background.
When init or body of a with() block fails check if the bkg()
process already exited and report its status (including stdout/
/stderr). This significantly improves debugability.
====================

Link: https://patch.msgid.link/20260223202633.4126087-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>