[test] Rework test_download to allow running all tests of a single extractor

[oddstr13]

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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)
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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

The goal is being able to run info extractor tests for only a single provider or extractor.
This makes the test usable when maintaining individual providers.

python -m unittest test.test_download.TestDownload.test_nrk runs extractor tests for the contents of extractor/nrk.py.
python -m unittest test.test_download.TestDownload.test_nrk.test_NRKTVEpisode runs extractor tests for NRKTVEpisodeIE.

TODO:

• Fix running all extractor tests with python test/test_download.py
• Update documentation

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This PR is currently marked as draft because it seems I may have broken running the test with python test/test_download.py
As only a single (erroring) test gets ran then.

Reviews are still welcome, both from maintainers and others with knowledge of python's testing features.