selftests: netdevsim: hw_stats_l3: Add a new test

Add a test that verifies basic UAPI contracts, netdevsim operation,
rollbacks after partial enablement in core, and UAPI notifications.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>