testing: fix time reported for failing tests.

t.ns was hanging after recent changes.

R=gri, rsc
CC=golang-dev
https://golang.org/cl/5237044

src/pkg/testing/testing.go

@@ -110,6 +110,7 @@ func (t *T) Failed() bool { return t.failed }
 // FailNow marks the Test function as having failed and stops its execution.
 // Execution will continue at the next Test.
 func (t *T) FailNow() {
+	t.ns = time.Nanoseconds() - t.ns
 	t.Fail()
 	t.ch <- t
 	runtime.Goexit()