os/exec: fix -test.run argument for new 'go test'

os/exec: fix -test.run argument for new 'go test'

In 'go test' I deleted the leading package. prefix
from all the test names, since it contained no actual
information.  Adjust the -test.run argument accordingly.
This will still work with the current gotest too, since
the argument is an unanchored pattern.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5491058