selftests: net: prevent Python from buffering the output

Make sure Python doesn't buffer the output, otherwise for some
tests we may see false positive timeouts in NIPA. NIPA thinks that
a machine has hung if the test doesn't print anything for 3min.
This is also nice to heave for running the tests manually,
especially in vng.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250716205712.1787325-1-kuba@kernel.org


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