selftests: net: udpgro_fwd.sh: explicitly checking the available ping feature

selftests: net: udpgro_fwd.sh: explicitly checking the available ping feature

As Paolo pointed out, the result of ping IPv6 address depends on
the running distro. So explicitly checking the available ping feature,
as e.g. do the bareudp.sh self-tests.

Fixes: 8b3170e07539 ("selftests: net: using ping6 for IPv6 in udpgro_fwd.sh")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Link: https://lore.kernel.org/r/825ee22b-4245-dbf7-d2f7-a230770d6e21@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>