cmd/go: clean test directories as they complete
cmd/go: clean test directories as they complete

A go build currently generates around 400MB of test output prior to
cleaning up. With this change we use a maximum of ~15MB.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5588044

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go