test/hardlink: introduce `fd` variable to hold the file open file descriptor

In preparation for adding a test that uses `AT_EMPTY_PATH` and will want
a reference to an existing open file descriptor refering to some file,
introduce a new variable to hold the return value of `open` so that
`ret` can be modified by the test without losing the number of the
file descriptor so it can be closed.

Signed-off-by: Charles Mirabile <cmirabil@redhat.com>