write script to clean up image files that have been generated by the tests

[tobiasschweizer]

the tests generate image files: test/_test_data/images/knora, test/_test_data/images/thumb, test/_test_data/images/tmp

I added an additional request, so the tmp file created by make_thumbnail actually gets created. The final file ends up in test/_test_data/images/knora. I will add another test for convert_from_binaries.

[macflav]

Fixing a typo (the text says "There", instead of "The").