test: Make run.go's errorcheck behave like testlib.
test: Make run.go's errorcheck behave like testlib.

testlib will complain about any unmatched errors left in errorchecks while run.go will not.

Fixes issue 4141.

R=golang-dev, minux.ma, remyoudompheng, rsc
CC=golang-dev
http://codereview.appspot.com/6614060
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go