test-copy: add a test shuffling bytes between normal files

I started looking into adding copy_file_range support, and discovered
that we can improve the way we call sendfile:
- sendfile(2) man page is missing an important bit: the number of bytes to
  copy cannot be too big (SSIZE_MAX actually), and the description of EINVAL
  return code does not mention this either,
- our implementation works but calls sendfile over and over with a small
  size, which seems suboptimal.

First add a test which (under strace) can be used to see current behaviour.

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