tools: ynl: add YNL test framework

tools: ynl: add YNL test framework

Add a test framework for YAML Netlink (YNL) tools, covering both CLI and
ethtool functionality. The framework includes:

1) cli: family listing, netdev, ethtool, rt-* families, and nlctrl
   operations
2) ethtool: device info, statistics, ring/coalesce/pause parameters, and
   feature gettings

The current YNL syntax is a bit obscure, and end users may not always know
how to use it. This test framework provides usage examples and also serves
as a regression test to catch potential breakages caused by future changes.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20251124022055.33389-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>