if_ovpn tests: fix ra test

Client one is supposed to be assigend 198.51.100.2, but sometimes it
loses the race and the address ends up assigned to client two. When this
happened one would try to ping itself, which failed because the loopback
interface isn't up.

Ideally we'd generate static address assignments, but that would
complicate the test. Activate loopback interfaces so the test always
passes, and just try to ping both possible addresses.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")