style: Reformat test_impersonated_credentials.py to fix black lint error

[amtk3]

Fixes a linting error reported by the CI runner (black formatting check failed).

Reformatted packages/google-auth/tests/test_impersonated_credentials.py to match the repository's black style guide guidelines, correcting line lengths and indentation in the test_sign_bytes_configures_mtls and test_id_token_refresh_configures_mtls tests.

Successfully verified by running nox -s lint and python3 -m pytest tests/test_impersonated_credentials.py locally.

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PR created automatically by Jules for task 3391227870716880661 started by @amtk3

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