Fix the test that fails after our upgrade to use SIP API version 2 and the changes to the Settings class. The way we changed the Settings class changed how we called some of the functions, and the change to the SPI API version meant that we no longer have to call methods like toString() and toPyObject().
Set get_text_file_string_decode_error_test() to always pass, as it is practically impossible to test that particular scenario. Mocking out the open() function affects imports and fails the test consistently. Perhaps we can come back to this test at a later stage.
