The favicon for testwikidata needs to have a 48x48px layer added to it, as it is only currently available in 16x16px and 32x32px versions (the only favicon to be so).
The file is located in the operations/mediawiki-config repo at mediawiki-config/docroot/bits/favicon/testwikidata.ico, and was added with Gerrit change 67980 by Reedy.
I'm unsure what is the original file used for the favicon, but for what it's worth, the original Wikidata logo can be found at https://commons.wikimedia.org/wiki/File:Wikidata-logo.svg.
There is a nice on-line service that lets you generate multi-layer .ico files from a single PNG at http://convertico.org/image_to_icon_converter/. Use it at your own risk :-)
Version: wmf-deployment
Severity: enhancement