selftests: net: report output format as TAP 13 in Python tests

selftests: net: report output format as TAP 13 in Python tests

The Python lib based tests report that they are producing
"KTAP version 1", but really we aren't making use of any
KTAP features, like subtests. Our output is plain TAP.

Report TAP 13 instead of KTAP 1, this is what mptcp tests do,
and what NIPA knows how to parse best. For HW testing we need
precise subtest result tracking.

Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250228180007.83325-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>