[Test] Improve download test

[dirkf]

Boilerplate: own code/improvement

Please follow the guide below

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Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This PR makes two small and one larger improvement to test/test_download.py, in 3 separate commits.

Calculating the file hash

External downloaders may not support limiting the download exactly to a specified size. When running a test download, only a short initial portion of the file is supposed to be downloaded, and the hash is calculated from that portion. If the portion is longer than requested the hash will be incorrect. The PR explicitly uses the requested length, if specified, when calculating the file hash, so that the same value should be calculated regardless of downloader.

Header-related params in test cases

Header-related params (here: --user-agent, --referer, --add-header) are parsed into std_headers rather than the params dict in the main yt-dl program. In download tests, only params that are set into the params dict were supported, and header-related params in test cases were ignored. This PR changes the params processing for test downloads to match what happens in the main program.

As an example, the change makes it possible to specify a test case with a specific UA, such as this for an Invidious site fronted by Cloudflare

        ...
        'params': {
            # Cloudflare breaks HTTP if Chrome is mentioned in the UA (2021-08)
            'user_agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/6533.18.5 (KHTML, like Gecko) Safari/6533.18.5',
        },                                       
        ...

Add TestSuite test_..._all with all downloads for each extractor

When testing downloads for an extractor XXX, a set of test cases is created but it's not obvious which item in the extractor's _TESTS list corresponds to which TestDownload.test_XXX... test case. If there are many test cases one might fail to run some of the later tests, or run past the highest numbered case.

To facilitate running the download tests for an extractor in the same way that they run in the CI servers, the PR adds a further download test case with name suffix _all for each extractor class that has test case(s). The new test case is a unittest.TestSuite that runs all the other download tests for that extractor. Counts of the possible test outcomes (error, failure, etc) are printed.

Thus to test YoutubeIE:

python test/test_download.py TestDownload.test_Youtube_all

Supersedes, closes #27461.

[dirkf]

Note to self:

• also handle case where a fragment is downloaded that is smaller than expected by the original code.