unix: remove usage of ioutil.TempFile in tests

Mostly change that to os.Create(filepath.Join(t.TempDir(), "filename"))
so that the cleanup is done automatically (and, because this is a new
directory, we do not need to have a "create unique temp file name"
logic).

While at it, fix some log/error messages -- if an error is coming from
e.g. os package, it is already wrapped, so there's no need to add more
context.

Change-Id: I62f13679f256b94095be5ca1e77a3fa302f01b97
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526298
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>

unix/xattr_test.go

@@ -8,8 +8,8 @@
 package unix_test
 
 import (
-	"io/ioutil"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"testing"
@@ -127,11 +127,10 @@ func TestXattr(t *testing.T) {
 }
 
 func TestFdXattr(t *testing.T) {
-	file, err := ioutil.TempFile("", "TestFdXattr")
+	file, err := os.Create(filepath.Join(t.TempDir(), "TestFdXattr"))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.Remove(file.Name())
 	defer file.Close()
 
 	fd := int(file.Fd())