Update test_compare_files() to use pytest syntax.
Update documentation.
Rename test_compare_files() -> test__compare_files().
Fix syntax error.
Replace test__compare_files() with test_unique_records_getter() & test_matching_records_getter()

Test the public API instead of the private member.
Close open_file when processing is complete.
Fix bug.

Bug was causing unique_records and matching_records to give incorrect output when performing multiple comparisons.