testutil: whitelist thread created by go cover

pkg/testutil/leak.go

@@ -106,6 +106,7 @@ func interestingGoroutines() (gs []string) {
 		}
 		stack := strings.TrimSpace(sl[1])
 		if stack == "" ||
+			strings.Contains(stack, "created by os/signal.init") ||
 			strings.Contains(stack, "runtime/panic.go") ||
 			strings.Contains(stack, "created by testing.RunTests") ||
 			strings.Contains(stack, "testing.Main(") ||