selftests: net: fix timeout passed as positional argument to communicate()

selftests: net: fix timeout passed as positional argument to communicate()

The cited commit refactored the hardcoded timeout=5 into a parameter,
but dropped the keyword from the communicate() call.
Since Popen.communicate()'s first positional argument is 'input' (not
'timeout'), the timeout value is silently treated as stdin input and the
call never enforces a timeout.

Pass timeout as a keyword argument to restore the intended behavior.

Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20260310115803.2521050-3-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>