tests: Fix assert_files_hardlinked
tests: Fix assert_files_hardlinked

It was always succeeding because we were trying to stat the inode number, and
failing, and thus getting the empty string for both, which compared as true.

Regression from:
<https://github.com/ostreedev/ostree/commit/74e3581e>

Noticed this while working on
<https://github.com/ostreedev/ostree/pull/974>
and looking at the test results.

Closes: #976
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree