if_ovpn tests: Exercise the multihome option

These test cases are variants of the 4in4 and 6in6 tests wherein the
server interface has an alias assigned and the client is configured to
connect to the alias rather than the primary address.

Reviewed by: kp
MFC after: 1 month
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51499

(cherry picked from commit 0bfcfb3cb1cbfa383cbd24eff39d39f143eb63ba)