Add basic ipdivert tests, checking:
* IPv4 writes to remote destination
* IPv6 writes to remote destination
* IPv4 writes to local system (based on IP address).
Note that the tests require `ipdivert` kernel module, which depends on `ipfw` kernel module.
Blindly loading `ipdivert` results in all connectivity being lost.
Given that, all these tests have side effect of setting `net.inet.ip.fw.default_to_accept=1` loader tunable and loading `ipfw` and `ipdivert` modules (if not already).