selftests: forwarding: ping{6, }_test(): Add description argument

Have ping_test() recognize an optional argument with a description of
the test. This is handy if there are several ping test, to make it clear
which is which.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>