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/syscall_unix_test.go

@@ -126,11 +126,10 @@ func TestSignalNum(t *testing.T) {
 
 func TestFcntlInt(t *testing.T) {
 	t.Parallel()
-	file, err := ioutil.TempFile("", "TestFcntlInt")
+	file, err := os.Create(filepath.Join(t.TempDir(), t.Name()))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.Remove(file.Name())
 	defer file.Close()
 	f := file.Fd()
 	flags, err := unix.FcntlInt(f, unix.F_GETFD, 0)
@@ -293,9 +292,9 @@ func passFDChild() {
 	// We make it in tempDir, which our parent will clean up.
 	flag.Parse()
 	tempDir := flag.Arg(0)
-	f, err := ioutil.TempFile(tempDir, "")
+	f, err := os.Create(filepath.Join(tempDir, "file"))
 	if err != nil {
-		fmt.Printf("TempFile: %v", err)
+		fmt.Println(err)
 		return
 	}
 
@@ -452,11 +451,10 @@ func TestSetsockoptString(t *testing.T) {
 }
 
 func TestDup(t *testing.T) {
-	file, err := ioutil.TempFile("", "TestDup")
+	file, err := os.Create(filepath.Join(t.TempDir(), t.Name()))
 	if err != nil {
-		t.Fatalf("Tempfile failed: %v", err)
+		t.Fatal(err)
 	}
-	defer os.Remove(file.Name())
 	defer file.Close()
 	f := int(file.Fd())