Make 'never_convert_lossy_files' tests more meaningful

We should expect the original file extensions to be preserved since we
do not transcode lossy files. The tests now fail, but a fix is coming
up.
convert: Change file extension only if actually transcoded

This makes the tests from a37cabb969edfcc34a6ec628b1bd97eda15eeedd
pass.
Extend and refactor  'never_convert_lossy_files' tests

* Each test function only uses on fixture, so we create fixtures in the
  test functions and not in the `setup()` method.

* We test the effect of the option set to true and false.
convert: add cli flag to skip confirmation

The flag mirrors the `--yes` flag from the modify command.
Swap 'if not' and 'else' for readability