selftests: forwarding: Add description to the multipath tests

Add a better description to the summary for multipath tests. e.g.,

INFO: Running IPv6 multipath tests
TEST: ECMP                                               [PASS]
INFO: Expected ratio 1.00 Measured ratio 1.02
TEST: Weighted MP 2:1                                    [PASS]
INFO: Expected ratio 2.00 Measured ratio 2.02
TEST: Weighted MP 11:45                                  [PASS]
INFO: Expected ratio 4.09 Measured ratio 4.03

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>