Add test file for JXS when the JXS implementation is released

[gabriel-vasile]

JXS is a very new image format and the decoder/encoder implementations have not been released.
According to Wikipedia, these will be released sometime in the future. https://en.wikipedia.org/wiki/JPEG_XS#Standards

#319 added support for JXS detection but didn't add a test file because it's impossible to find/create one.
This issue is a reminder to add a test file when they are available.