unix: fix finalizer fd close bug in TestOpenByHandleAt

The raw fd is successively wrapped using os.NewFile, so dont defer close
the raw fd, but instead use (*os.File).Close.

Fixes golang/go#49403

Change-Id: I1f4b12536ff0abea6b1680a4ff932b7eefa57162
Reviewed-on: https://go-review.googlesource.com/c/sys/+/361995
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_test.go

@@ -720,10 +720,10 @@ func TestOpenByHandleAt(t *testing.T) {
 			if err != nil {
 				t.Fatalf("OpenByHandleAt: %v", err)
 			}
-			defer unix.Close(fd)
-
 			t.Logf("opened fd %v", fd)
 			f := os.NewFile(uintptr(fd), "")
+			defer f.Close()
+
 			slurp, err := ioutil.ReadAll(f)
 			if err != nil {
 				t.Fatal(err)