OK, just revisited running this myself, and I assume my head-scratching may be the source of your problem. Ref.:
env PATH=$PATH:/Users/bep/dev/go/bep/godartsass/lib/sass_embedded-beta6 go test -run TestSC ./hugolib
The sass_embedded-beta6 (or beta7) folder above is what needs to be on the PATH (not the binary/script).