selftests: net: print full exception on failure

Instead of a summary line print the full exception.
This makes debugging Python tests much easier.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240412141436.828666-5-kuba@kernel.org


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