refactor(tests): add helper function for tests

[l-lumin]

Introduce a helper function validate_strings to streamline the validation of expected and excluded strings in test cases. Replace repetitive string assertions in the test_markitdown_local function with calls to this new helper, improving code readability and maintainability.

root@261c16c64651:/workspaces/markitdown# hatch test
========================================================================================================== test session starts ==========================================================================================================
platform linux -- Python 3.13.1, pytest-8.3.4, pluggy-1.5.0
rootdir: /workspaces/markitdown
configfile: pyproject.toml
plugins: rerunfailures-14.0, mock-3.14.0, xdist-3.6.1
collected 3 items                                                                                                                                                                                                                       

tests/test_markitdown.py ..s                                                                                                                                                                                                      [100%]

=========================================================================================================== warnings summary ============================================================================================================
../../root/.local/share/hatch/env/virtual/markitdown/2jxz10qj/hatch-test.py3.13/lib/python3.13/site-packages/speech_recognition/__init__.py:7
  /root/.local/share/hatch/env/virtual/markitdown/2jxz10qj/hatch-test.py3.13/lib/python3.13/site-packages/speech_recognition/__init__.py:7: DeprecationWarning: aifc was removed in Python 3.13. Please be aware that you are currently NOT using standard 'aifc', but instead a separately installed 'standard-aifc'.
    import aifc

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================================================================== 2 passed, 1 skipped, 1 warning in 14.14s ================================================================================================
root@261c16c64651:/workspaces/markitdown# 

[gagb]

Good catch. Thank you!