selftests: drv-net: rss_ctx: Add test for ntuple rule

Extend the rss_ctx test suite to test that an ntuple action that
redirects to an RSS context contains that information in `ethtool -n`.
Otherwise the output from ethtool is highly deceiving. This test helps
ensure drivers are compliant with the API.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://patch.msgid.link/759870e430b7c93ecaae6e448f30a47284c59637.1732748253.git.dxu@dxuuu.xyz


Signed-off-by: Jakub Kicinski <kuba@kernel.org>