tfc testing - requires netkey

This uses tfc=0 which we can only really see when we look at recorded
tcpdump showing ESP packets around 120 bytes. Currently, test cases
do not yet store sanitized tcpdump output for verification.

We also set send-no-esp-tfc=yes on west to prevent east from using
padding. East is configured to use padding up to 1500 bytes.

tcpdump should show no padding is used be either east or west, so
ping encrypted ESP should be about 120 bytes.
