Following input from @fedorov , we tweaked the wording to mention that using the Slicer prefix was recommended but not mandatory. See update to the ci script and wiki.
The module download elastix binaries from our server and use them in its logic class. Is there anything I should to make things work? I run the testing in differentWindow and Ubuntu machines and it works. The extension does not support Mac for now but mac users are welcome to modify it and I can help in this if needed (I don’t have mac).
If you need Elastix, please do not download binaries from your own server. Instead, add SlicerElastix to EXTENSION_DEPENDS in your CMakeLists.txt file. When you install your extension, the extension manager will also install SlicerElastix extension, which has all the Elastix binaries on all platforms (including MacOS) and compatible with Slicer (uses Slicer’s ITK). Using the proper binaries might also fix your module tests.