Exhaustive socket policy tests.

This change contains two things. A simple version of "@parameterized"
testing, and a single socket policy test that runs with 120 different
parameter combinations.

The result is that xfrm_test.py now covers basic socket policy
functionality for every combination of,
- encryption
- authentication
- IP version
- transport protocol

Bug: 63589916
Test: net_test passing on all kernels except 3.18
Change-Id: I11179e26dd26de64c089199a398db90891d028c2