Log the error's underyling errno if there is

This test occasionally fails on Windows. This commit logs extra
information about the error to understand the situation better.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

server_test.go

@@ -25,6 +25,7 @@ import (
  "runtime"
  "strings"
  "sync"
+ "syscall"
  "testing"
  "time"
 
@@ -369,6 +370,13 @@ func TestClientEOF(t *testing.T) {
  if err := client.Call(ctx, serviceName, "Test", tp, tp); err == nil {
  t.Fatalf("expected error when calling against shutdown server")
  } else if !errors.Is(err, ErrClosed) {
+ errno, ok := err.(syscall.Errno)
+ if ok {
+ t.Logf("errno=%d", errno)
+ } else {
+ t.Logf("error %q doesn't match syscall.Errno", err)
+ }
+
  t.Fatalf("expected to have a cause of ErrClosed, got %v", err)
  }
 }