This refactoring will allow using `TestParser._do_parse_test()` method for subclasses defined in inheriting modules.
This way, if the new module defines new files in other paths than `account_statement_import_camt/test_files/`,
subclasses of `TestParser` will still be able to use the `_do_parse_test()` method for imports and checks.
Moreover, a custom value can now be defined for the acceptable differing lines number between inputfile and goldenfile.
It will also maintain backward compatibility with the current usage of the method
(ie: using only a filename with the assumption of finding it in `account_statement_import_camt/test_files/`).
NB: methods are now defined outside of `TestParser` to allow importing them directly, instead of inheriting from `TestParser`.
