exp/ssa/interp: disable interp_test on non-POSIX.
exp/ssa/interp: disable interp_test on non-POSIX.

R=gri, rsc
CC=golang-dev
https://codereview.appspot.com/7422051

apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,dsriram/golang