Fix tests

aggregate_tests.py now runs every unit test in any file named test_*.py in tuf/tests, running them as a single suite. To accomodate this, the unit tests no longer run themselves when imported, and any print statements they had generated have been replaced with logs. quickstart.py is now in tuf/repo, where test_quickstart.py can reliably find it. thest_hash.py no longer runs tests using both hashlib and pycrypto if one of the two is not available.