selftests: mlxsw: Add target for router test on spectrum

IPv4 routes in Spectrum are based on the kvd single-hash, but as it's
a hash we need to assume we cannot reach 100% of its capacity.

Add a wrapper that provides us with good/bad target numbers for the
Spectrum ASIC.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
[petrm@mellanox.com: Drop shebang.]
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>