testing: use runtime/debug to format panics

Sorry, Mercurial failed me.

TBR=r, dsymonds
CC=golang-dev
https://golang.org/cl/5649080

src/pkg/testing/testing.go

@@ -248,13 +248,11 @@ func tRunner(t *T, test *InternalTest) {
 	// a call to runtime.Goexit, record the duration and send
 	// a signal saying that the test is done.
 	defer func() {
-		if false {
 		// Log and recover from panic instead of aborting binary.
 		if err := recover(); err != nil {
 			t.failed = true
 			t.Logf("%s\n%s", err, debug.Stack())
 		}
-		}
 
 		t.duration = time.Now().Sub(t.start)
 		t.signal <- t