The test suite was actually doing something before, just
not quite what I intended. Without `-U` for bare-user checkouts
we end up doing a copy.
Now, a future commit will change how rofiles work, which would cause
the test suite to permit inplace mutation for non-hardlinked files.
So let's ensure they *are* hardlinked.
Closes: #462
Approved by: jlebon
mkdir mnt
-$OSTREE checkout test2 checkout-test2
+$OSTREE checkout -U test2 checkout-test2
rofiles-fuse checkout-test2 mnt
cleanup_fuse() {