[WIP] Refactor test_updater.py and repository_setup.py.

Add generate.py script and pre-generated repository files used by Unit tests (replacing repository_setup.py).
Refactor test_updater.py (10 test cases remaining).
[WIP] Refactor test_updater.py

Continue refactoring the test cases of test_updater.py.
Fix issue where repository_tool.py was not writing new compressed metadata.
Minor edits to TUF modules.