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

src/pkg/os/exec/exec_test.go

@@ -18,7 +18,7 @@ import (
 )
 
 func helperCommand(s ...string) *Cmd {
-	cs := []string{"-test.run=exec.TestHelperProcess", "--"}
+	cs := []string{"-test.run=TestHelperProcess", "--"}
 	cs = append(cs, s...)
 	cmd := Command(os.Args[0], cs...)
 	cmd.Env = append([]string{"GO_WANT_HELPER_PROCESS=1"}, os.Environ()...)