test-rofiles-fuse: Actually check out via hardlinks

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

tests/test-rofiles-fuse.sh

@@ -30,7 +30,7 @@ echo "1..6"
 
 mkdir mnt
 
-$OSTREE checkout test2 checkout-test2
+$OSTREE checkout -U test2 checkout-test2
 
 rofiles-fuse checkout-test2 mnt
 cleanup_fuse() {