Add Input Checking to the VTI tests

Verify the input path of the VTI by twisting packets
from the VTI and sending them back in to the other
side of the same transform.

Bug: 68223648
Test: parallel_tests.sh 20 20 xfrm_tunnel_test.py on android-3.18
      android-4.4 and android-4.9
Change-Id: I52b513cdafdb47543bc27332051ee84d3e97bb0b