go.tools/go/vcs: do not delete $TMPDIR during test runs
go.tools/go/vcs: do not delete $TMPDIR during test runs

The test would nuke the entire contents of os.TempDir on completion.

This change corrects the code to use ioutil.TempDir.

R=r, adg
CC=golang-dev
https://codereview.appspot.com/12796045

karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,chenjianlong/go.tools,lattera/go.tools,karmakaze/go.tools,lattera/go.tools