Add missing test cases to test_util.py.

Add missing test cases for the following functions:
get_target_hash()
find_delegated_role()
paths_are_consistent_with_hash_prefixes()
ensure_all_targets_allowed()

Add test condition to test_B6_load_json_string().
Merge branch 'develop' of https://github.com/theupdateframework/tuf into add_test_cases_to_test_util
Merge branch 'add_test_cases_to_test_util' of https://github.com/vladimir-v-diaz/tuf into add_test_cases_to_test_util
Remove old 'tuf/tests/unit/test_repository' directory.
Resolve Merge.
Merge pull request #191 from vladimir-v-diaz/add_test_cases_to_test_util

Add missing test cases to test_util.py.