Write better unit tests for dirserv_load_fingerprint_file()
The existing tests don't catch format string bugs, because they only run on expected inputs: https://github.com/torproject/tor/pull/970#pullrequestreview-242268332
The existing tests don't catch format string bugs, because they only run on expected inputs: https://github.com/torproject/tor/pull/970#pullrequestreview-242268332