testing: report test as failed if the test panics.

Fixes #5149.

R=golang-dev, dave, minux.ma
CC=golang-dev
https://golang.org/cl/8136043

src/pkg/testing/testing.go

@@ -337,6 +337,7 @@ func tRunner(t *T, test *InternalTest) {
 		t.duration = time.Now().Sub(t.start)
 		// If the test panicked, print any test output before dying.
 		if err := recover(); err != nil {
+			t.Fail()
 			t.report()
 			panic(err)
 		}