security/openvpn: fix test suite when ifconfig emits ::1/128 address format

Some systems apparently format output of ifconfig lo0 similar to
"inet6 ::1/128" instead of 12.1's "inet6 ::1 prefixlen 128". This
confuses the test script, so strip the slash and trailing prefixlen
off.

Since that bug affects the build-time test suite and its occurrence
breaks the build, no PORTREVISION bump needed.

Reported by: des@