test/run: use correct $PWD to make os.Getwd less expensive
test/run: use correct $PWD to make os.Getwd less expensive

The commands being run are 'go tool this' and 'go tool that',
and the go command will call Getwd during its init.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7336045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang