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
@rochan In my opinion if this message is a part of functionality - w should check it, or, if you want to wxtract this from that test - write another for just a notification message