app/test: fix IPv6 header initialization

app/test: fix IPv6 header initialization

Fix two issues found when writing PMD unit tests for HW ptype and
L4 checksum offload:

   - The version field in the IPv6 header was being set to zero,
     which prevented hardware from recognizing it as IPv6. The
     IP version field is now set to six.
   - The payload_len field was being initialized using host byte
     order, which (among other things) resulted in incorrect L4
     checksum computation. The payload_len field is now set using
     network (big-endian) byte order.

Fixes: 92073ef961ee ("bond: unit tests")
Cc: stable@dpdk.org
Signed-off-by: Lance Richardson <lance.richardson@broadcom.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>