Add test coverage for delegated hash bins target download

[ivanayov]

This change adds tests coverage for path_hash_prefixes and
verifies that role names matching specific prefixed successfully
find and download the corresponding metadata files and do not
succeed to find existing, but non-matching files

Fixes #1639

Follow-up of #1808

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• [n/a] Docs have been added for the bug fix or new feature

[coveralls]

Pull Request Test Coverage Report for Build 2027942163

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.312%

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Totals
Change from base Build 2022257279:	0.0%
Covered Lines:	1179
Relevant Lines:	1195

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💛 - Coveralls

[jku]

I've not given this the review it needed, sorry... This vaguely looks like we could do better but I can't immediately see the better way: did you consider having the same test setup as in the existing downloads test test_targetfile_search() -- so there would be

• staticly defined metadata structure: it could be the same delegations_tree already defined in the file, just added with a targets metadata that does hashed bin delegations
• test cases would then only have to define a target path to search for, search result and the expected visited order

This would not make the current test much shorter but it might mean that adding a new test case would be maybe 2 lines instead of 40 lines of test data that it is now?

[MVrachev]

I just noticed you are not using visited_order anywhere in your test.

[lukpueh]

Is this superseded by #2031? Should we think about deprecating path_hash_prefixes now that TAP 15 has landed? (see see theupdateframework/taps#132 (comment))

[jku]

Is this superseded by #2031?

I think so. I wouldn't object to a tight client test specifically for hashed bins but I don't think it's super important, and this PR is 80 lines of test code for very limited (new) test functionality.