Add gtpu flow offload test suite

The implementation of GTPU offload rx is to use ip4_gtpu flow MARK action
of NIC and vpp flow REDIRECT_TO_NODE and BUFFER_ADVANCE fuction to direct
received gtpu flow to gtpu4-flow-input graph node, skipped ethernet-input,
ip4-input, ip4-lookup, ip4-local, ip4-udp-lookup normal graph node
processing.

Verified on 3n-clx and Intel E810 NIC environment, single core with 64B packet,
performance improve ~33% that compare with pure software way.

Signed-off-by: xinfeng zhao <xinfengx.zhao@intel.com>
Signed-off-by: Yulong Pei <yulong.pei@intel.com>
Change-Id: I2af4589448bdb1729e4ce206a8cf3a1239c61af8
Signed-off-by: Yulong Pei <yulong.pei@intel.com>