testutil: whitelist WaitGroup.Done

Calling a WaitGroup.Done() in a defer will sometimes trigger the leak
detector since the WaitGroup.Wait() will unblock before the defer
block completes. If the leak detector runs before the Done() is
rescheduled, it will spuriously report the finishing Done() as a leak.
This happens enough in CI to be irritating; whitelist it and ignore.

pkg/testutil/leak.go

@@ -118,6 +118,7 @@ func interestingGoroutines() (gs []string) {
 		}
 		stack := strings.TrimSpace(sl[1])
 		if stack == "" ||
+			strings.Contains(stack, "sync.(*WaitGroup).Done") ||
 			strings.Contains(stack, "created by os/signal.init") ||
 			strings.Contains(stack, "runtime/panic.go") ||
 			strings.Contains(stack, "created by testing.RunTests") ||