UploadFromUrlTest: Don't reference skins/common/ (via bits.wm.o)

Files from that directory have a tendency to suddenly disappear.

Replaced the URL with the MediaWiki logo used on mediawiki.org, which
should be not disappear. This whole stuff is a bit meh though.

Change-Id: I7bf22bd7e51f518500e5767b4d5012c2eb17f020
Merge "UploadFromUrlTest: Don't reference skins/common/ (via bits.wm.o)"