cmd/go: fix go test -compiler
cmd/go: fix go test -compiler

The compiler must be changed with the Set method
so that the buildToolchain gets updated too.

Fixes issue 3231.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5768044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang