selftests: net: Add test cases for link and peer netns

selftests: net: Add test cases for link and peer netns

- Add test for creating link in another netns when a link of the same
   name and ifindex exists in current netns.
 - Add test to verify that link is created in target netns directly -
   no link new/del events should be generated in link netns or current
   netns.
 - Add test cases to verify that link-netns is set as expected for
   various drivers and combination of namespace-related parameters.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Link: https://patch.msgid.link/20250219125039.18024-14-shaw.leon@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>