encoding/xml: flush buffer after encoding token

R=rsc, bradfitz, adg
CC=golang-dev
https://codereview.appspot.com/13004046

Committer: Andrew Gerrand <adg@golang.org>
testing: don't start timing a Parallel test until it's actually starting
Fixes issue 5285.

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/13045044