selftests: mlxsw: hw_stats_l3: Add a new test

Add a test that verifies that UAPI notifications are emitted, as mlxsw
installs and deinstalls HW counters for the L3 offload xstats.

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