selftests: add a test for ethtool pause stats

Make sure the empty nest is reported even without stats.
Make sure reporting only selected stats works fine.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>