selftests: net: py: extract the case generation logic

selftests: net: py: extract the case generation logic

In preparation for adding test variants move the test case
collection logic to a dedicated function. New helper returns

 (function, args, name, )

tuples. The main test loop can simply run them, not much
logic or discernment needed.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251120021024.2944527-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>