Update themeunittestdata.wordpress.xml with CORS-accessible images

[dd32]

This PR attempts to replace the image URLs in the WXR with a variant that will be importable into Playground.

This has been untested as-is. This is provided such that others can give it a try. The WXR url to be used would be this:

https://raw.githubusercontent.com/WordPress/theme-test-data/try/cors-accessible-images/themeunittestdata.wordpress.xml

See https://github.com/WordPress/theme-test-data/issues/82#issuecomment-2055567664 for the idea behind this.

See #82