Refactor unittest tests to use pytest

[Honza-m]

Proposed change

There was one last left over unittest.TestCase class testing the secret substitution in config files. This PR refactores the existing tests class into a set of pytest tests that test the same functionality. I've moved this into a new file as it seemed like a nice group of tests to still keep together.

The tests should be functionally identical to the original, although the test setup has been reworked to be a bit more explicit.

The only functional change is in test_secrets_logger_removed, because the original didn't provide a secrets.yaml file at all and so resulted in a different HomeAssistanError than what I believe was expected. The pytest.raises statements now include a match keyword to check exactly what error is being raised.

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [ ] New feature (which adds functionality to an existing integration)
• [ ] Deprecation (breaking change to happen in the future)
• [ ] Breaking change (fix/feature causing existing functionality to break)
• [x] Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #40855
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• [x] The code change is tested and works locally.
• [x] Local tests pass. Your PR cannot be merged unless tests pass
• [x] There is no commented out code in this PR.
• [x] I have followed the development checklist
• [x] I have followed the perfect PR recommendations
• [x] The code has been formatted using Ruff (ruff format homeassistant tests)
• [ ] Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• [ ] Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• [ ] The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• [ ] New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• [ ] For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.

To help with the load of incoming pull requests:

• [ ] I have reviewed two other open pull requests in this repository.