cmd/go: test: clean up all temporary directories

go test may call builder.init() multiple times which will create a new work directory.  The cleanup needs to hoist the current work directory.
Fixes #7904.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/95900044

src/cmd/go/build.go

@@ -447,7 +447,8 @@ func (b *builder) init() {
 			fmt.Fprintf(os.Stderr, "WORK=%s\n", b.work)
 		}
 		if !buildWork {
-			atexit(func() { os.RemoveAll(b.work) })
+			workdir := b.work
+			atexit(func() { os.RemoveAll(workdir) })
 		}
 	}
 }