selftests: net: py: add test case filtering and listing

selftests: net: py: add test case filtering and listing

When developing new test cases and reproducing failures in
existing ones we currently have to run the entire test which
can take minutes to finish.

Add command line options for test selection, modeled after
kselftest_harness.h:

  -l       list tests (filtered, if filters were specified)
  -t name  include test
  -T name  exclude test

Since we don't have as clean separation into fixture / variant /
test as kselftest_harness this is not really a 1 to 1 match.
We have to lean on glob patterns instead.

Like in kselftest_harness filters are evaluated in order, first
match wins. If only exclusions are specified everything else is
included and vice versa.

Glob patterns (*, ?, [) are supported in addition to exact
matching.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Tested-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260410013921.1710295-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>