test/fixed-reuse: test that linked file slot reuse is correct

If we do:

<open fileX into slot 0>
<open fileY into slot 0><read slot 0><close slot 0>

this should be consistent with not having opened fileX first, and the
read should be satisfied from fileY.

Signed-off-by: Jens Axboe <axboe@kernel.dk>