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.