Improve test suite

I'm currently working on improving the test suite.

I only changed the current tests to work with the new setup.
More tests need to be added.

Tests that I know need to be in there:
* Shrink (for all below the end result need to have the correct dimensions)
  * Without aspect ratio
    * without upsizing
    * with upsizing
  * With aspect ratio
    * without upsizing
    * with upsizing
* Crop
  * Wrong file type
  * Cropped size is correct

Not sure what other test right now.

I'll keep working in this pull request locally to add the tests, if want to wait 'till these test are implemented let me know and I'll update it here when I'm finished.