selftests: drv-net: add a trivial ping test

Add a very simple test for testing with a remote system.
Both IPv4 and IPv6 connectivity is optional, later change
will add checks to skip tests based on available addresses.

Using netdevsim:

 $ ./run_kselftest.sh -t drivers/net:ping.py
 TAP version 13
 1..1
 # timeout set to 45
 # selftests: drivers/net: ping.py
 # KTAP version 1
 # 1..2
 # ok 1 ping.test_v4
 # ok 2 ping.test_v6
 # # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0
 ok 1 selftests: drivers/net: ping.py

Command line SSH:

 $ NETIF=virbr0 REMOTE_TYPE=ssh REMOTE_ARGS=root@192.168.122.123 \
    LOCAL_V4=192.168.122.1 REMOTE_V4=192.168.122.123 \
    ./tools/testing/selftests/drivers/net/ping.py
 KTAP version 1
 1..2
 ok 1 ping.test_v4
 ok 2 ping.test_v6 # SKIP Test requires IPv6 connectivity
 # Totals: pass:1 fail:0 xfail:1 xpass:0 skip:0 error:0

Existing devices placed in netns (and using net.config):

 $ cat drivers/net/net.config
 NETIF=veth0
 REMOTE_TYPE=netns
 REMOTE_ARGS=red
 LOCAL_V4="192.168.1.1"
 REMOTE_V4="192.168.1.2"

 $ ./run_kselftest.sh -t drivers/net:ping.py
 TAP version 13
 1..1
 # timeout set to 45
 # selftests: drivers/net: ping.py
 # KTAP version 1
 # 1..2
 # ok 1 ping.test_v4
 # ok 2 ping.test_v6 # SKIP Test requires IPv6 connectivity
 # # Totals: pass:1 fail:0 xfail:1 xpass:0 skip:0 error:0

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240420025237.3309296-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>