Test that dst cache is cleared with socket policy.

This is a small test to exercise a known kernel bug. XFRM policy
can be ignored if sk_dst_cache is not cleared properly, so this
test does the following.
1. populates sk_dst_cache
2. applies per-socket xfrm policy
3. tests whether that policy is applied to traffic

Test: run_net_test.sh xfrm_test.py
Bug: 63589547
Change-Id: I8e81423d0ef833fd935dcbd7f7180a0a37ed9dbb