if_ovpn tests: skip float and linklocal test on < 2.7

Earlier versions (than 2.7.0) do not support float notifications or link-local
addresses. Skip the relevant tests there.

PR: 289150
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D52234

(cherry picked from commit 512d3c3a2c6391cee61ff41e3fe6304973346857)