carp tests: Basic functionality test

Set up three vnet jails, bridged together. Run carp between two of them.
Attempt to provoke locking / epoch issues.

Reviewed by: mav (previous version), melifaro, asomers
Differential Revision: https://reviews.freebsd.org/D24303

(cherry picked from commit 47308803e7b92c15326707f76739bcb6dc1383c7)