Refactor test_slow_retrieval_server.py.

Refactored to use the 'unittest' module (test conditions in code, rather
than verifying text output), use pre-generated repository files, and
discontinue use of the old repository tools. Expanded comments and modified
previous setup.

Minor edits to slow_retrieval_server.py.

Add missing else clauses (to detect when TUF fails to prevent an attack) for a couple of the integrations tests.
Merge pull request #200 from vladimir-v-diaz/refactor_test_slow_retrieval_attack

Refactor test_slow_retrieval_server.py.