selftests: forwarding: Introduce $ARPING

Instead of relying on "arping" being installed everywhere under that
name, introduce a variable $ARPING like the other tools do.

Convert an existing test, mirror_gre_vlan_bridge_1q.sh to
require_command $ARPING and then invoke arping through the variable.

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