QAART 412: Fixing the ImageStorage test

The main change is to use dedicated test files instead of random image file from the Special:NewFile page. The problems with using random file is that if a file with long revision history is chosen, the test would file.

https://wikia-inc.atlassian.net/browse/QAART-412
Remember, there is a possibility this images might be deleted by a test (wich is deleting random image)