ngclient testing: client workflow sequences

Description of issue or feature request:

If needed, narrow down the scope of this issue and spawn separate ones for top-level metadata update, delegations metadata update, Fetching targets.

edit: This issue deals with top-level-metadata updates. Delegated roles and targets will be covered #1641 #1642 #1643 .

Following the high-level steps described in #1579 implement tests for the various client workflow sequences:

• Tests scenarios from specification (Can we mark these to be our conformance tests?)
• Specific sequences the client should be able to handle go here
• Extracted useful scenarios from legacy tests
• Specific python-tuf sequences (for ex. loading and validating local metadata)

Decide what we want to test sometimes and what should be always included:

• errors / no errors
• what metadata is downloaded?
• what metadata is read from disk?
• what metadata is on disk afterwards?
• are the currently loaded metadata versions correct?
• (some of the same bullet points repeated for targets)
• ? more ?

The tests should be RepositorySimulator based.

[sechkova]

I opened two follow up issues for testing the rest of the steps following the client workflow from the specification: #1641 #1642. We can close this one.

[jku]

The new set of tests (test_updater_top_level_update.py) are in my opinion the state of the art now:

• tests are reasonably short and readable
• it’s easy to see what individual tests actually check

If we can document the decision making process that lead to these choices (situation X -> should test for Y) maybe in the test module docstring or somewhere that would be good… but if there are no generic rules and we just need to look at individual tests, then I guess the current examples are good examples to look at.

Can we close this or are there more sub issues that need to be filed first?