ngclient Updater: write tests to handling coverage report

[MVrachev]

Description of issue or feature request:
This is the minimal amount of testing we should have to enable coverage for the ngclient updater and the first step of resolving #1462 for the Updater.
When resolving the missing lines reported by coverage we will decide if we want to handle all of them
or there is a sense of leaving some of them.

[MVrachev]

We should remember to test downloading files with filenames HASH.FILENAME.EXT or when consistent_snapshot is True.

[MVrachev]

The current coverage all percentage reported for tuf/api/ngclient/updater.py is 77 %.

[joshuagl]

From @MVrachev in #1463 (review)

there are a couple of code paths not tested yet (according to coverage all):

delegation role with terminating option turn on
test when a target is not found
reaching the max limit of visiting delegations, but there are more delegations left
Additionally, we never test DelegatedRole.is_in_trusted_paths() when path_hash_prefixes is set and thus never actually test calculating the hashes.

[jku]

also

• we're probably not testing cycle prevention in _preorder_depth_first_walk(): this is the if (role_name, parent_role) in visited_role_names branch
• test that terminating actually prevents backtracking to other roles (but does not prevent e.g. finding an earlier sibling role)

[sechkova]

Up to now the coverage has been/will be improved by:

• ng client improve TrustedMetadataSet testing #1477
• tests: Add some missing coverage #1553
• More trusted set tests #1522
• Remove max_length parameter from fetch #1519

I am assigning this to myself to finish the work and include ngclient to the coverage report.