net: disable multicast test on Alpha GNU/Linux
net: disable multicast test on Alpha GNU/Linux

Alpha GNU/Linux does not have /proc/net/igmp[6].

R=golang-dev, rsc, bradfitz
CC=golang-dev
http://codereview.appspot.com/5663047

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