fix random EBADF problem

```ruby
      r, w = IO.pipe
      r.close

      IO.for_fd(w.fileno).close
```

This code closes a file descriptor `w.fileno`, but `w` doesn't know
the closing. Another code can open same file descriptor with opening
file (`f`). After that, the `w` will GCed and `w.fileno` is closed
again, and `f.fileno` is closed too, so IO operations for `f` (`f.close`)
will cause EBADF.

To fix this issue, do this test in another process.

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