server: wait longer in afterTest

Because we sets dial timeout to be 200ms, afterTest should wait
> 200ms.

etcd/z_last_test.go

@@ -75,7 +75,7 @@ func afterTest(t *testing.T) {
 		").noteClientGone(":                            "a closenotifier sender",
 	}
 	var stacks string
-	for i := 0; i < 4; i++ {
+	for i := 0; i < 6; i++ {
 		bad = ""
 		stacks = strings.Join(interestingGoroutines(), "\n\n")
 		for substr, what := range badSubstring {