selftests: net: py: explicitly forbid multiple ksft_run() calls
People (do people still write code or is it all AI?) seem to not
get that ksft_run() can only be called once. If we call it
multiple times KTAP parsers will likely cut off after the first
batch has finished.
Link: https://patch.msgid.link/20260408221952.819822-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>