Simon, I'm following up here instead of at https://bugs.debian.org/1081519 because I didn't think we needed to copy the Release Team on this and it's a more specific issue.
These build tests backed by Mutter have only been enabled starting in July. We didn't know that we weren't running them, but we could consciously choose not to run them now. They won't run if mutter and related Build-Depends marked with <!nocheck> and architecture qualifiers aren't installed at build time. One negative of using Mutter for gnome-settings-daemon build tests is that gnome-settings-daemon is used by several other desktops like GNOME Flashback which we expect would be easier for some architectures to get working than Mutter. That's why I was ok with not running these tests on many architectures anyway. Do you want me to basically just undo my test enablement from July or do you want to use a different approach? Thank you, Jeremy Bícha