selftests: net: py: add test variants

selftests: net: py: add test variants

There's a lot of cases where we try to re-run the same code with
different parameters. We currently need to either use a generator
method or create a "main" case implementation which then gets called
by trivial case functions:

  def _test(x, y, z):
     ...

  def case_int():
     _test(1, 2, 3)

  def case_str():
     _test('a', 'b', 'c')

Add support for variants, similar to kselftests_harness.h and
a lot of other frameworks. Variants can be added as decorator
to test functions:

  @ksft_variants([(1, 2, 3), ('a', 'b', 'c')])
  def case(x, y, z):
     ...

ksft_run() will auto-generate case names:
  case.1_2_3
  case.a_b_c

Because the names may not always be pretty (and to avoid forcing
classes to implement case-friendly __str__()) add a wrapper class
KsftNamedVariant which lets the user specify the name for the variant.

Note that ksft_run's args are still supported. ksft_run splices args
and variant params together.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20251120021024.2944527-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>