tests: create unit tests for verifying the protection against rollback attacks

[rdimitrov]

This is a follow-up issue raised during the discussion of No protection against rollback attacks for roles other than root.

The goal is to update or create new test cases which verify the rollback attack protection against the different roles in TUF - root, timestamp, snapshot, targets and delegated targets.

References:

• Link to the advisory describing the reason for the implementation - GHSA-66x3-6cw3-v5gj
• Link to the implementation - ed6788e
• Link to the documentation describing the expected behavior - TUF