Skip chflags tests in filetests.t on dragonflybsd

DragonflyBSD's file tests don't seem to be chflags-aware:

  $ touch foo
  $ if [ -w foo ]; then echo "Yes"; fi
  Yes
  $ chflags uchg foo
  $ if [ -w foo ]; then echo "Yes"; fi
  Yes

versus FreeBSD:

  $ touch foo
  $ if [ -w foo ]; then echo "Yes"; fi
  Yes
  $ chflags uchg foo
  $ if [ -w foo ]; then echo "Yes"; fi
  $