compiler: fix testing.Main invocation in extension_test.go
compiler: fix testing.Main invocation in extension_test.go

R=dsymonds
CC=golang-dev
http://codereview.appspot.com/4312062

Committer: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

andrewzeneski/goprotobuf,polydawn-ports/goprotobuf,supiyun/goprotobuf,ruiaylin/goprotobuf,VividCortex/goprotobuf,wfxiang08/goprotobuf,adwinsky/goprotobuf,liumuqi/goprotobuf,tonnerre/go-protobuf,t3rm1n4l/goprotobuf,go-mirrors/goprotobuf