refactor: Deprecated ImportTestCase in favor of PytestImportHelper

[semohr]

Description

This PR introduces PytestImportHelper which replaces ImportTestCase. I tried to propagate all updates already, this PR touches:

• test_scrub
• test_filefilter
• test_replaygain
• test_chroma
• test_import: Major refactor to use pytest

I know this is quite a big PR, the changes are very related tho. The only thing missing to allow us to remove the ImportTestCase is to migrate all tests using AutotagImportTestCase.

TODOs:

• Changelog Not needed as this is an internal refactor only

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This is related to the multi-step efforts to improve logging in beets #6553

[codecov]

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