use UTF-8 encoding in unit test test_output_to_file

[oxygen-dioxide]

On my Chinese windows, this test fails with the following error:

        with open(output_file, "r") as f:
>           output_data = f.read()
                          ^^^^^^^^
E           UnicodeDecodeError: 'gbk' codec can't decode byte 0xaa in position 61: illegal multibyte sequence

tests\test_cli_vectors.py:87: UnicodeDecodeError

because Markitdown writes to file in UTF-8, but the test tries to read it in local encoding