TODO: Include time override checks in unit tests in Primary and Secondary

[awwad]

Given the merge of upTUF PR 22, some adjustments to testing are merited. While this code is all covered via higher-level functions, there are more specific tests that should be added:

• In test_primary and test_secondary, for the unit tests for each module's validate_time_attestation() function, check that the TUF time override has been updated appropriately. Pause a few seconds and make sure the time remains the same (TUF time override).
• In the unit tests for Primary.primary_update_cycle() (and possibly the Secondary equivalent), make sure that override times get updated appropriately.